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THE EVOLUTION OF WORLDS 



THE MACMILLAN COMPANY 

NEW YORK • BOSTON • CHICAGO 
ATLANTA • SAN FRANCISCO 

MACMILLAN & CO., Limited 

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THE MACMILLAN CO. OF CANADA, Ltd. 

TORONTO 




Saturn — photographed at the Lowell Observatory 
BY Mr. E. C. Slipher, September, 1909. 



THE EVOLUTION OF 

WORLDS 



BY 



PERCIVAL LOWELL, A.B., LL.D. 

AUTHOR OF "mars AND ITS CANALS," "MARS AS TUE 

ABODE OF LIFE," ETC. 

DIRECTOR OF THE OBSERVATORY AT FLAGSTAFF, ARIZONA; NON-RESIDENT PROFESSOR 

OF ASTRONOMY AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY; FELLOW OF 

THE AMERICAN ACADEMY OF ARTS AND SCIENCES; MEMBRE DE LA SOCIE'TE 

a.stkonomique de france; member of the astronomical and astro- 
physical society of america; mitglied der astronomische ge- 
sellschaft; membre de la societe belge d'astronomie; 
honorary member of the sociedad astronomica de mex- 
ICO; JANSSEN medallist OF THE SOCIETE ASTRONOMIQUE DE 
FRANCE, 1904, FOR RESEARCHES ON MARS; MEDALLIST 
OF THE SOCIEDAD ASTRONOMICA DE MEXICO 
FOR STUDIES ON MARS, 1908 



ILLUSTRATED 



THE MACMILLAN COMPANY 

1909 

^11 rights renewed 







c^^ 



\.^ 



Copyright, 1909, 
By the MACMILLAN COMPANY. 



Set up and electrotyped. Published December, 1909. 



NortoooK i^ress 

J. 8. Gushing Co. — Berwick & Smith Co. 

Norwood, Mass., U.S.A. 



iCI.A2530^B 



Co 



THE PRESIDENT OF THE MASSACHUSETTS INSTITUTE 

OF TECHNOLOGY 

TO MY COLLEAGUES THERE 

AND TO ITS STUDENT BODY 

TO WHOSE INTEREST AND ATTENTION THESE 

LECTURES ARE INDEBTED 

VhEY ARE APPRECIATIVELY INSCRIBED 



** Si je n'etais pas devenu general en chef et I'instrument du sort d'un 
grand people, j'aurais couru les bureaux et les salons pour me mettre 
dans la dependance de qui que ce fut, en qualite de ministre ou d'am- 
bassadeur ? Non, non! je me serais jete dans 1' etude des sciences 
exactes. J'aurais fait mon chemin dans la route des Galilee, des 
Newton. Et puisque j'ai reussi constamment dans mes grandes entre- 
prises, eh bien, je me serais hautement distingue aussi par des travaux 
scientifiques. J'aurais laisse le souvenir de belles decouvertes. Aucune 
autre gloire n'aurait pu tenter mon ambition." 

— Napoleon i^^, quoted by Arago. 



The substance of the following pages was written 
and presented in a university course of lectures before 
the Massachusetts Institute of Technology — in Feb- 
ruary and March of this year. The kind interest 
with which the lectures were received, not only by the 
students and professional bodies, but by the public, 
was followed by an immediate request from The Mac- 
millan Company to issue them in book form, and as 

such they now appear. 

PERCIVAL LOWELL. 

Boston, Mass., May 29, 1909. 



Vll 



CONTENTS 



CHAPTER 

I. Birth of a Solar System .... 

II. Evidence of the Initial Catastrophe in Our Own 

III. The Inner Planets . . . . 

IV. The Outer Planets . . . . 
V. Formation of Planets .... 

VI. A Planet's History — Self-sustained Stage 

VII. A Planet's History — Sun-sustained Stage . 

VIII. Death of a World ..... 



PAGE 


I 


N 

31 


. 58 


. 94 


. 127 


. 155 


. 182 


. 213 



NOTES 

1. Meteor Orbits ... . . • 

2. Densities of the Planets ..... 

3. Variation in Spectroscopic Shift 

4. On the Planets' Orbital Tilts 

5. Planets and their Satellite Systems 

6. On the Induced Circularity of Orbits through 

Collision . . . . . 

7. Capture of Satellites ..... 

Index ......... 



241 

243 

243 
244 

245 

250 
251 

253 



IX 



LIST OF ILLUSTRATIONS 



I. 
IL 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

X. 

XI. 

XII. 



PLATES 
Saturn ....... Frontispiece 

OPPOSITE PACE 

The Moving Nebula surrounding Nova Persei, 1901 — 



1902 .... 
Representative Stellar Spectra . 
Spectra of the Major Planets . 
Venus, I 896-1 897 
Asteroids : Major Axes of Orbits 
Saturn — A Drawing showing Agglomerations 
Spectrogram of Jupiter, Moon Comparison 



Spectrogram showing Water- vapor in Atmosphere of Mars 1 60 
Tree Fern . . . . . . . 

Ten Views of Mercury, showing Effect of Libration 
Spectrogram of Saturn . . . . 



CUTS APPEARING IN TEXT 

Algol and its Dark Companion .... 
Nova Persei . . . 

Spectrum of Nova Persei . . . . . 

The Moving Nebula surrounding Nova Persei, 1901 
Great Nebula in Orion . . . , . 

Great Nebula in Andromeda .... 
Nebula M. 100 Comae ..... 
Nebula J^i I. 226 Urs^ Majoris 

Nebula ^ V. 24 Comae. Showing Globular Structure 

xi 



H 

24 

52 
82 

98 

108 

152 



176 

222 
232 

PAGE 

4 
1 1 

12 

13 

17 
18 

19 

20 

21 



xu 



LIST OF ILLUSTRATIONS 



Nebula M. loi Ursae Majoris .... 

The Radiant of a Meteoric Shower 

Diagram explaining Proportionate Visibility of Meteors 

The Mart Iron ...... 

Section of Meteorite showing Widmannstattian Lines 
Meteorite, Toluca ...... 

Nebula ifi V. 14 Cygni . . 

Nebula N.G.C. 1499 Persei .... 

Nebula N.G.C. 6960 in Cygnus 

Nebula M. 51 Canum Venaticorum . . . 

Orbits of the Inner Planets .... 

Sulla Rotazione di Mercurio. — Di G. V. Schiaparelli 
Map of Mercury. Lowell .... 

Venus. October, 1896-March, 1897 

Venus. April 12, 1909. .... 

Diagram : Convection Currents in Atmosphere of Venus 
Diagram : Shift in Central Barometric Depression . 
Spectrogram of Venus, showing its Long Day 



Spectrogram of Jupiter, giving the Length of 

of its Spectral Lines . 
Orbits of the Outer Planets 
Drawing of Jupiter showing its Ellipticity 
Two Drawings of Jupiter and its Wisps 
Photograph of Jupiter, 1909 
Diagram of Saturn's Rings 
The Tores of Saturn 
Chart showing increasing Tilts of the Major Planets 
Orbital Tilts and Eccentricities of Satellites . 
Masses of Planets and Satellites . . . . 

Two Drawings of Jupiter and its ** Great Red Spot 
Sun Spots . . . . . , 

Photograph of a Sun Spot . . . . 



its Day by the Tilt 



PAGE 

37 
38 

41 

42 

43 

45 
46 

47 
48 

59 
64 

69 

78 

79 
81 

81 

87 

89 
95 

^3 

05 

07 

13 
H 
31 

33 
36 
64 

65 
66 



LIST OF ILLUSTRATIONS . xiii 



PAGE 



The Volcano Colima, Mexico, March 24, 1903 . . .169 

Jukes Butte, a Denuded Laccohth, as seen from the Northwest. 170 
Ideal Section of a Laccolith . . . . . .170 

Earth as seen from above. — Photographed at an Altitude of 

5500 Feet ..... . . .183 

Tracks of Sauropus Primaevus . . . . . .188 

Adventures of a Heat Ray . . . . , .193 

Polar Caps of Mars at their Maxima and Minima . . .198 

Glacial Map of Eurasia ....... 200 

Map showing the Glaciated Area of North America . .201 

Photograph of the Moon ....... 205 

Petrified Bridge, Third Petrified Forest, near Adamana, Arizona 2 i o 
Three Views of Venus, showing Agreement at Diiferent Distances 220 
Diagram of Libration in Longitude due to Rotation . . 222 

Moon, — Full and Half . . . . , , .225 

Diagram illustrating Molecular Motion in a Gas . . .227 

Distribution of Molecular Velocities in a Gas , . . 229 



THE EVOLUTION OF WORLDS 

CHAPTER I 

BIRTH OF A SOLAR SYSTEM 

ASTRONOMY is usually thought of as the study 
of the bodies visible in the sky. And such it 
largely is when the present state of the universe alone 
is considered. But when we attempt to peer into its 
past and to foresee its future, we find ourselves facing a 
new side of the heavens — the contemplation of the 
invisible there. For in the evolution of worlds not 
simply must the processes be followed by the mind's 
eye, so short the span of human life, but they begin 
and end in what we cannot see. What the solar 
system sprang from, and what it will eventually become, 
is alike matter devoid of light. Out of darkness into 
darkness again: such are the bourns of cosmic action. 
The stars are suns; past, present, or potential. Each 
of those diamond points we mark studding the heavens 
on a winter's night are globes comparable with, and 
in many cases greatly excelling, our own ruler of the 
day. The telescope discloses myriads more. Yet 



/^ 



2 THE EVOLUTION OF WORLDS 

these self-confessed denizens of space form but a 
fraction of its occupants. Quite as near, and perhaps 
much nearer, are orbs of which most of us have no 
suspicion. Unimpressing our senses and therefore 
ignored by our minds, bodies people it which, except 
for rare occurrences, remain forever invisible. For 
dark stars in countless numbers course hither and 
thither throughout the universe at speeds as stupendous 
as the lucent ones themselves. 

Had we no other knowledge of them, reasoning 
would suffice to demonstrate their existence. It is 
the logic of unlimited subtraction. Every self-shining 
star is continually giving out light and heat. Now 
such an expenditure cannot go on forever, as the source 
of its replenishing by contraction, accretion, or dis- 
integration is finite. Long to our measures of time as 
the process may last, it must eventually have an end 
and the star finally become a cold dark body, pursuing 
as before its course, but in itself inert and dead ; an 
orb grown orbed, in the old French sense. So it must 
remain unless some cosmic catastrophe rekindle it to 
life. The chance of such occurrence in a given time 
compared with the duration of the star's light-emitting 
career will determine the number of dark stars relative 
to the lucent ones. The chance is undoubtedly small, 
and the number of dark bodies in space proportionally 
large. Reasoning, then, informs us first that such 



BIRTH OF A SOLAR SYSTEM 3 

bodies must exist all about us, and second that their 
multitude must be great. 

Valid as this reasoning is, however, we are not left 
to inference for our knowledge of them. There is 
a certain star amid the polar constellations known 
as Algol, — ^ el Ghoul, the Arabs called it, or The 
Daemon. The name shows they noticed how it winked 
its eye and recognized something sarcastically sinister 
in its intent. For once in two days and twenty hours 
its light fades to one-third of its usual amount, remains 
thus for about twenty minutes, and then slowly regains 
its brightness. Seemingly unmoved itself, its steady 
blinking from the time man first observed it took on 
an uncanniness he felt. To untelescoped man it 
certainly seemed demoniacal, this punctual recurrent 
wink. Spectroscoped man has learnt its cause. 

Goodricke in 1795 divined it, and research since has 
confirmed his keen intuition. Its loss of light is oc- 
casioned by the passing in front of it of a dark com- 
panion almost of its own size revolving about it in a 
close elliptic orbit. That this is the explanation of 
its strange behavior, the shift of its spectral lines makes 
certain, by showing that the bright star is receding 
from us at twenty-seven miles a second seventeen hours 
before the eclipse and coming towards us at about the 
same rate seventeen hours after it; its dark companion, 
therefore, doino; the reverse. 



4 THE EVOLUTION OF WORLDS 

Algol is no solitary specimen of a mind-seen invisible 
star. Many eclipsing binaries of the same class are 
now known; and considering that the phenomenon 
could not be disclosed unless the orbital plane of the 

Algol and its dark companion, 




AS SEEN FROM THE EARTH, 




P. L. 



AS SEEN FROM ABOVE ORBIT. 



pair traversed the observer's eye, an unlikely chance 
in a fortuitous distribution, we perceive how many such 
in truth there must be which escape recognition for 
their tilt. 

But if dark stars exist in connection with lucent ones, 



BIRTH OF A SOLAR SYSTEM 5 

there must be many more that travel alone. Our own 
Sun is an instance in embrj^o. If he Hve long enough, 
he will become such a solitary shrouded tramp in his 
old age. For he has no companion to betray him. 
The only way in which we could become cognizant of 
these wanderers would be by their chance collision 
with some other star, dark or lucent as the case might 
be. The impact of the catastrophe would generate 
so much light and heat that the previously dark body 
would be converted into a blazing sun and a new star 
make its advent in the sky. 

Star births of the sort have actually been noted. 
Every now and then a new star suddenly appears in 
the firmament — a nova as it is technically called. 
These apparitions date from the dawn of astronomic 
history. The earliest chronicled is found in the Chinese 
Annals of 134 B.C. It shone out in Scorpio and was 
probably the new star which Pliny tells us incited 
Hipparchus, "The Father of Astronomy," to make his 
celebrated catalogue of stars. From this time down 
we have recorded instances of like character. 

One of the most famous was the "Pilgrim Star" of 
Tycho Brahe. That astronomer has left us a full ac- 
count of it. "While I was living," he tells us, **with my 
uncle in the monastery of Hearitzwadt, on quitting my 
chemical laboratory one evening, I raised my eyes to 
the well-known vault of heaven and observed, with in- 



6 THE EVOLUTION OF WORLDS 

describable astonishment, near the zenith, in Cassiopeia, 
a radiant fixed star of a magnitude never before seen. 
In my amazement I doubted the evidence of my senses. 
However, to convince myself that it was no illusion, and 
to have the testimony of others, I summoned my assist- 
ants from the laboratory and inquired of them, and of 
all the country people that passed by, if they also ob- 
served the star that had thus suddenly burst forth. I 
subsequently heard that in Germany wagoners and 
other common people first called the attention of as- 
tronomers to this great phenomenon in the heavens, — 
a circumstance which, as in the case of non-predicted 
comets, furnished fresh occasion for the usual raillery 
at the expense of the learned." 

The new star, he informs us, was just like all other 
fixed stars, but as bright as Venus at her brightest. 
Those gifted with keen sight could discern it in the day- 
time and even at noon. It soon began to wane. In 
December, 1572, it resembled Jupiter, and a year and 
three months later had sunk beyond recognition to the 
naked eye. It changed color as it did so, passing from 
white through yellow to red. In May, 1573, it returned 
to yellow ("the hue of Saturn," he expressly states), 
and so remained till it disappeared from sight, scintillat- 
ing strongly in proportion to its faintness. 

Thirty-two years later another stranger appeared and 
was seen by Kepler, who wrote a paper about it en- 



BIRTH OF A SOLAR SYSTEM 7 

titled "The New Star in the Foot of the Serpent." It 
shone out in the same sudden manner and faded in the 
same leisurely way. 

Since i860 there have been several such apparitions, 
and since 1876 it has been possible to study them with 
the spectroscope, which has immensely increased our 
knowledge of their constitution. Indeed, this instru- 
ment of research has really opened our eyes to what they 
are. Nova Cygni, in 1876, Nova Aurigae, in 1892, and 
Nova Persei, in 1901, besides several others found by 
Mrs. Fleming on the Arequipa plates, were excellent 
examples, and all agreed in their main features, showing 
that novae constitute a type of stars by themselves, 
whose appearing in the first place and whose behavior 
afterwards prove them to have started from like cause 
.and to have pursued parallel lines of development. 

As a typical case we may review the history of Nova 
Aurigae. On February i, 1892, an anonymous post- 
card was received by Dr. Copeland of the Royal Ob- 
servatory, Edinburgh, that read as follows: "Nova in 
Aurigae. In Milky Way, about 2° south of ^ Aurigae, 
preceding 26 Aurigae. Fifth magnitude slightly brighter 
than ^." The observatory staff at once looked for the 
nova and easily found it with an opera-glass. They 
then examined it through a prism placed before their 
24-inch reflector and found its spectrum. It proved to 
be that of a "blaze star." 



8 THE EVOLUTION OF WORLDS 

Dr. Thomas D. Anderson turned out to be the writer 
of the anonymous post-card — his name modestly self- 
obhterated by the nova's hght. He had detected the 
star on January 24, but had only verified it as a new one 
on the 31st. Harvard College Observatory then looked 
up its archived plates. The plates showed that it had 
appeared sometime between December i and 10. Its 
maximum had been attained on December 20, after 
which it declined, to record apparently another maxi- 
mum on February 3 of the 3.5 magnitude. From this 
time its light steadily waned till on April i it was only 

of the 1 6th magnitude or of what it had been. 

I 00000 

In August it brightened again and then waned once 
more. 

Meanwhile its spectrum underwent equally strange 
fluctuations. At first it exhibited the bright lines 
characteristic of the flaming red solar prominences, 
the calcium, hydrogen, and helium lines flanked by 
their dark correlatives upon a continuous background, 
showing that both glowing and cooler gases were here 
concerned. The sodium lines, too, appeared, like those 
that come out in comets as they approach the furnace 
of the Sun. An outburst such as occurs in miniature in 
the solar chromosphere or outermost gaseous layer of 
the Sun was here going on upon a gigantic scale. A 
veritable spectral chaos next supervened, staying until 



BIRTH OF A SOLAR SYSTEM 9 

the star had practically faded away. Then, on its re- 
appearance, in August, Holden, Schaeberle, and Camp- 
bell discovered to their surprise not what had been at all, 
but something utterly new : the soberly bright lines only 
of a nebula. Finally, ten years later, January, 1902, 
Campbell found its spectrum had become continuous, 
the body having reverted to the condition of a star. 

Now how are we to interpret these grandiose vicissi- 
tudes, visually and spectrally revealed .? That we wit- 
nessed some great catastrophe is clear. The sudden 
increase of light of many thousand fold from invisibility 
to prominence shows that a tremendous cataclysm oc- 
curred. The bright lines in the spectrum confirm 
it and imply that vast upheavals like those that shake 
the Sun were there in progress, but on so stupendous a 
scale that, if for no other reason, we must dismiss the 
idea that explosions alone can possibly be concerned. 
The dark correlatives of the bright lines have been in- 
terpreted as indicating that two bodies were concerned, 
each travelling at velocities of hundreds of miles a 
second. But in Nova Aurigae shiftings of the spectral 
lines impljang six bodies at least were recorded, if such 
be attributed to motion in the line of sight, and Vogel 
was minded to throw in a few planets as well — as Miss 
Clerke pithily puts it. There is not room for so many 
on the stage of the cosmic drama. Other causes, as we 
now know, may also displace the spectral lines. Great 



10 THE EVOLUTION OF WORLDS 

pressure has been shown to do it, thanks to the labors 
of Humphreys and Mohler at Baltimore. "Anomalous 
refraction " may do it, as Professor Julius of Utrecht has 
found out. Finally, changes of density may produce 
it, as Michelson has discovered. To these causes we 
may confidently ascribe most of the shiftings in the 
stellar spectrum, for just such forces must be there at 
work. 

Mr. Monck suggested the idea that new stars are the 
result of old dark stars rushing through gaseous fields 
in space and rendered luminous by the encounter. See- 
liger revived and developed this idea, which in certain 
cases is undoubtedly the truth. Probably this occurred 
to the new star of 1885 which suddenly blazed out 
almost in the centre of the great nebula in Andromeda. 
It behaved like a typical nova and in due course faded 
to indistinguishability. Something like it happened, 
too, in the nova of i860, which suddenly flared up in 
the star cluster 80 Messier, outdoing in lustre the cluster 
itself, and then, too, faded away. 

But just as psychology teaches us that not only do we 
cry because we are sorrowful, but that we are sorrowful 
because we cry, so while a nova may be made by a 
nebula, no less may a nebula be made by a star. 

Let us see how this might be brought about and what 
sign manuals it would present. Suppose that the two 
bodies actually grazed. Then the disruption would 



BIRTH OF A SOLAR SYSTEM 



ir 



affect the star's cuticle, first raising the outer parts, con- 
sisting rather of carbon than of the metals, since that 
substance is the lighter, to intense heat and the gases 
about it at the same time. The glowing carbon would 
be intensely bright, and at first its light would over- 
power that from the gases, and not till its great glow had 











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which the more solid particles due to the disruption 
would fall together and light up again by their individ- 
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striking the stars passed close. 



12 



THE EVOLUTION OF WORLDS 



. Now to put this theory to the proof. In the early 
morning of the 22d of February, 1901, Dr. Anderson, 
the discoverer of Nova Aurigae, perceived that Algol 
had a neighbor, a star as bright as itself, which had 

Hr/H^KHe HS' 



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Feb. 27 



Feb. 28 



Mar. 6 



Mar. IS 



Mar. 28 






Spectrum of Nova Persei. (F. EUerman, 40 in. Yerkes.) 

never been there before. Within twenty-four hours 
of its detection the newcomer rivalled Capella, and 
shortly after took rank as the premier star of the north- 
ern hemisphere. Its spectrum on the 22d was found 
at Harvard College Observatory to be like that of Rigel, 
a continuous one crossed by some thirty faint dark lines. 



BIRTH OF A SOLAR SYSTEM 



13 



On the 24th, however, so soon as tt began to wane, the 
bright lines of hydrogen were conspicuous with their 
dark correlatives, just as they had been with Nova 
Aurigae and other novae. At the same time each par- 
ticular spectral line proved a law unto itself, some 
shifted more than others, thus negativing motion as 



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their only cause and indicating change of pressure or 
density as concerned concomitants of the affair. Blue 
emissions like those of Wolf-Rayet stars next made their 
appearance; then a band, found by Wright at the Lick 
to characterize nebulae, shone out, and finally in July the 
change to a nebular spectrum stood complete. 

Then came what is the most suggestive feature in 
the whole event. On August 22 and 23 Dr. Wolf at 



14 THE EVOLUTION OF WORLDS 

Konigstahl took with his then new Bruce objective 
some long exposure plates of the nova, and on them 
found, to his surprise, wisps of nebulous matter to the 
southeast of the star. On September 20 Ritchey, 
with a two-foot mirror of his own constructing exposed 
for four hours, brought the whole formation to light. 
It turned out to be a spiral nebula encircling and ap- 
parently emanating from the star. Its connection with 
the nova was patent. But there was more to come. 
Later plates taken at the Lick on November 7 dis- 
closed the startling fact that the nebula was visibly 
expanding, uncoiling outward from the star. A plate 
by Ritchey on November 13 confirmed this, and still 
later plates by him in December, January, and Febru- 
ary showed the motion to be progressive. At the same 
time the star showed no parallax, and the speed of the 
motion seemed thus to be indicated as enormous. 
Kapteyn suggested to account for it that appearance,. 
not reality, was here concerned; that the nebula had 
always existed, and was only shown up by the light 
from the conflagration travelling outward from the 
nova at the rate of one hundred and eighty-six thousand 
miles a second. This would make the catastrophe to 
have occurred as far back as the tim.e of James I, of 
which the news more truthful but less timely than that 
of the morning papers had only just reached us. 

But a little of that simple reasoning by which Zadig 



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BIRTH OF A SOLAR SYSTEM 15 

recovered the lost horses of the Sultan, and which from 
its unaccustomedness in the affairs of men got him sus- 
pected of having stolen them and very nearly caused 
his death, will show the untenableness of this idea and 
help us to a solution. In the first place we note that 
the star holds the very centre of the nebular stage, a 
remarkable prominence if the star has no creative right 
to the position. Then the same knots and patches of 
the nebulous configuration are visible in all the photo- 
graphs, in the same relative positions, turned through 
corresponding angles as one will see for himself, all 
having moved sj^mmetrically from one date to another. 
At the truly marvellous mimicry implied if different 
objects were concerned common sense instinctively 
shies, and very properly, as the chances against it are 
millions to one. Clearly it was not a mere matter 
of ethereal motion, but a very material motion of 
matter, which was here concerned. Something corpus- 
cular emanating from the nova spread outward into 
space. 

Clinching this conclusion is the result of a search by 
Perrine for traces of the nebula on earlier plates. For 
on one taken by him on March 29 (1901) he found the 
process already started in two close coils, its conception 
thus clearly dating from, the time of the star's outburst. 
In Nova Persei, then, we actually witnessed a spiral 
nebula evolved from a disrupted star. 



1 6 THE EVOLUTION OF WORLDS 

What was this ejectum and what drove it forth ? 
Professor Very regarded it as composed of corpuscles 
such as give rise to cathode rays discharged from the 
star under the stress of hght pressure or electric repul- 
sion. But I think we may see in it something simpler 
still; to wit, gaseous molecules driven off by light 
pressure alone — the smoke, as one may say, of the 
catastrophe — akin exactly to the constituents of com- 
et's tails. The mere light of the conflagration pushed 
the hydrogen molecules away. This would explain 
their presence and their exceeding hurry at the same 
time. They were started on their travels by domestic 
jars and kept going by the vivid after-effects of that 
infelicity. 

The fairly steady rate of regression from the nova 
observed may be explained by the observed decrease in 
the light of the repellent source. Such combined with 
the retarding effect of gravity might make the regres- 
sion equable. This is the more explanatory as the 
speed was certainly much less than that of light, though 
greatly exceeding any possible from the direct disrup- 
tion. At the same time both the bright and the dark 
lines of hydrogen seen in the spectrum stand accounted 
for; the colliding molecules, at their starting on their 
travels from the star, shining through their sparser 
fellows farther out. An interesting biograph of the 
levity of light ! 



BIRTH OF A SOLAR SYSTEM 



17 



Nova Persei thus introduces us at its birth to one of 
a class of most interesting objects comparatively re- 




Great Nebula in Orion — after Ritchey. 

cently discovered and of most pregnant import, — the 
spiral nebulae. 

In 1843 when Lord Rosse's giant speculum, six feet 



i8 



THE EVOLUTION OF WORLDS 



across, was turned upon the sky, a nebula was brought 
to Hght which was unHke any ever before seen. It was 




Great Nebula in Andromeda — after Ritchey. 

neither irregular like the great nebula in Orion nor round 
like the so-called planetary nebulae, — the two great 
classes at that time known, — but exhibited a striking 



BIRTH OF A SOLAR SYSTEM 



19 



spiral structure. It proved the forerunner of a remark- 
able revelation. For the specimen thus disclosed has 
turned out to typify not only the most interesting form 
of those heavenly wreaths of light, but by far the com- 
monest as v^ell. As telescopic and especially photo- 




NeBULA M. 100 COM^ — ■ AFTER ROBERTS. 

graphic means improved, the number of such objects 
detected steadily increased until about thirteen years 
ago Keeler by his systematic discoveries of them came 
to the conclusion that a spiral structure pervaded the 
great majority of all the nebulae visible. Their relative 
universality W2ls outdone only by the invariability of 
their form. For they all represent spirals of one type: 



20 THE EVOLUTION OF WORLDS 

two coiled arms radiating diametrically from a central 
nucleus and dilating outward. Even nebulae not 
originally supposed spiral have disclosed on better reve- 
lation the dominant form. Thus the great nebula in 
Andromeda formerly thought lens-shaped proves to be 




Nebula i^ I. 226 Urs^ Majoris — after Roberts. 

a huge spiral coiled in a plane not many degrees in- 
clined to the plane of sight. 

As should happen if the spirals are unrelated, left- 
handed and right-handed ones are about equally com- 
mon. In Dr. Roberts' great collection of those in 
which the structure is distinctly discernible, nine are 
right-handed, ten left-handed, showing that they par- 
take of the ambidextrous impartiality of space. 

Lastly the spirals are evidently thicker near the 



BIRTH OF A SOLAR SYSTEM 21 

centre, thinning out at the edge, and when the central 
nucleus is pronounced, it seems to have a certain globu- 
larity not shared by the arms, and more or less de- 
tached from them. This appears in those cases where 




Nebula i^ V. 24 Com^ — after Roberts. 
Showing globular structure. 

they are shown us edgewise, and it has been thought 
perceptible in the great nebula of Andromeda. The 
difficulty in establishing the phenomenon comes from 
the impossibility of both features showing at their best 
together. For the globularity to come out well, the 
spiral must be presented to us nearly in the plane of 
sight; for the spirality, in a plane at right angles to it. 



22 THE EVOLUTION OF WORLDS 

Much may be learnt by pondering on these pecuH- 
arities. The widespread character of the phenomenon 
points to some universal law. We are here clearly 
confronted by the embodiment of a great cosmic princi- 
ple, causing the helices it is for us to uncoil. It is a 
problem in mechanics. 

In the first place, a spiral structure denotes action 
on the face of it. It implies a rotation combined 
with motion out or in. We are familiar with the 
fact in the sparks of pin-wheel pyrotechnics. Any 
rotating fluid urged by an outward or an inward 
impulse must take the spiral form. A common ex- 
ample occurs in the water let out of a basin through a 
hole in the centre when we draw out the plug. Here 
the force is inward, and because the bowl and orifice 
are not perfectly symmetric, a rotation is set up in the 
water trying to escape, and the two combine to give us 
a beautiful conchoidal swirl. In this case the particles 
seek the centre, but the same general shape is assumed 
when they seek to leave it. 

Another point to be noticed is that a spiral nebula 
could not develop of itself and subsist. To continue 
it must have outside help. For if it were due to in- 
ternal explosive action in the pristine body, each ejec- 
tum must return to the point it started from, or else 
depart forever into space, for the orbit it would de- 
scribe must either be closed or unclosed. If the former, 



BIRTH OF A SOLAR SYSTEM 



23 



it would revisit its starting-point; if the latter, it would 
never return. Explosion, therefore, of itself could not 




Nebula M. ioi Urs^ Majorts — after Ritchey. 

have produced the forms we see, unless they be 
ephemeral apparitions, a supposition their presence 
throughout the heavens seems effectually to exclude. 



24 THE EVOLUTION OF WORLDS 

The form of the spiral nebulae proclaims their mo- 
tion, but one of its particular features discloses more. 
For it implies the past cause which set this motion going. 
A distinctive detail of these spirals, which so far as we 
know is shared by all of them, are the two arms which 
leave the centre from diametrically opposite sides. 
This indicates that the outward driving force acted 
only in two places, the one the antipodes of the other. 
Now what kind of force is capable of this peculiar 
effect .? If we think of the matter, we shall realize that 
tidal action would produce just this result. We see 
it daily in the case of the Moon; when it is high tide 
in the open ocean hereabouts, it is high tide also at the 
opposite end of the Earth. The reason is that the tide- 
raising body pulls the fluid nearest it more strongly 
than it pulls the Earth as a whole, and pulls the Earth 
as a whole more than it pulls the fluid at the opposite 
extremity. 

Suppose, now, a stranger to approach a body in space 
near enough ; it will inevitably raise tides in the other's 
mass, and if the approach be very close, the tides will be 
so great as to tear the body in pieces along the line due 
to their action; that is, parts of the body will be sepa- 
rated from the main mass in two antipodal directions. 
This is precisely what we see in the spiral nebula. Nor 
is there any other action that we know of which would 
thus handle the body. If it were to disintegrate under 



n 



o 



n/ 



BIRTH OF A SOLAR SYSTEM 25 

increased speed of rotation due to contraction upon it- 
self, parts of its periphery should be shed continually 
and a pinwheel of matter, not a two-armed spiral, be 
thrown off. If explosion were the disintegrating cause, 
disruption would occur unsymmetrically in one or more 
directions, not symmetrically as here. 

As the stranger passed on, his effect would diminish 
until his attraction no longer overbalanced that of the 
body for its disrupted portions. These might then be 
controlled and forced to move in elliptic orbits about 
the mass of which they had originally made part. 
Thence would come into being a solar system, the knots 
in the nebula going to form the planets that were to be. 

Before proceeding to what proof we have that it 
actually did occur in this way we may pause to consider 
some consequences of what we have already learned. 
Thus what brought about the beginning of the system 
may also compass its end. If one random encounter 
took place in the past, a second is as likely to occur in 
the future. Another celestial body may any day run 
into the Sun, and it is to a dark body that we must look 
for such destruction, because they are so much more 
numerous in space. 

That any of the lucent stars, the stars commonly so 
called, could collide with the Sun, or come near enough 
to amount to the same thing, is demonstrably impos- 
sible for aeons of years. But this is far from the case 



26 THE EVOLUTION OF WORLDS 

for a dark star. Such a body might well be within a 
hundredth of the distance of the nearest of our known 
neighbors, Alpha Centauri, at the present moment 
without our being aware of it at all. Our senses could 
only be cognizant of its proximity by the borrowed light 
it reflected from our own Sun. Dark in itself, our own 
head-lights alone would show it up when close upon us. 
It would loom out of the void thus suddenly before the 
crash. 

We can calculate how much warning we should have 
of the coming catastrophe. The Sun with its retinue 
is speeding through space at the rate of eleven miles a 
second toward a point near the bright star Vega. Since 
the tramp would probably also be in motion with a speed 
comparable with our own, it might hit us coming from 
any point in space, the likelihood depending upon the 
direction and amount of its own speed. So that at the 
present moment such a body may be in any part of the 
sky. But the chances are greatest if it be coming from 
the direction toward which the sun is travelling, since 
it would then be approaching us head on. If it were 
travelling itself as fast as the Sun, its relative speed of 
approach would be twenty-two miles a second. 

The previousness of the warning would depend upon 
the stranger's size. The warning would be long ac- 
cording as the stranger was large. Let us assume it 
the mass of the Sun, a most probable supposition. 



BIRTH OF A SOLAR SYSTEM 27 

Being dark, it must have cooled to a solid, and its density 
therefore be much greater than the Sun's, probably 
something like eight times as great, giving it a diameter 
about half his or four hundred and thirty thousand miles. 
Its apparent brightness would depend both upon its 
distance and upon its intrinsic brightness or albedo, and 
this last would itself vary according to its distance from 
the Sun. While it was still in the depths of space and 
its atmosphere lay inert, owing to the cold there, its 
intrinsic brightness might be that of the Moon or Mer- 
cury. As its own rotation would greatly affect the 
speed with which its sunward side was warmed, we can 
form no exact idea of the law of its increase in light. 
That the augmentation would be great we see from the 
behavior of comets as they approach the great hearth 
of our solar system. But we are not called upon to 
evaluate the question to that nicety. We shall assume, 
therefore, that its brilliancy would be only that of the 
Moon, remembering that the last stages of its fateful 
journey would be much more resplendently set off. 

With these data we can find how long it would be 
visible before the collision occurred. As a very small 
telescopic star it would undoubtedly escape detection. 
It is not likely that the stranger would be noticed simply 
from its appearance until it had attained the eleventh 
magnitude. It would then be one hundred and forty- 
nine astronomical units from the Sun or at five times 



28 THE EVOLUTION OF WORLDS 

the distance of Neptune. But its detection would come 
about not through the eye of the body, but through the 
eye of the mind. Long before it could have attracted 
man's attention to itself directly its effects would have 
betrayed it. Previous, indeed, to its possible showing in 
any telescope the behavior of the outer planets of the 
system would have revealed its presence. The far plum- 
met of man's analysis would have sounded the cause of 
their disturbance and pointed out the point from which 
that disturbance came. Celestial mechanics would have 
foretold, as once the discovery of another planet, so now 
the end of the world. Unexplained perturbations in 
the motions of the planets, the far tremors of its coming, 
would have spoken to astronomers as the first heralding 
of the stranger and of the destruction it was about to 
bring. Neptune and Uranus would begin to deviate 
from their prescribed paths in a manner not to be ac- 
counted for except by the action of some new force. 
Their perturbations would resemble those caused by an 
unknown exterior planet, but with this difference that the 
period of the disturbancewould be exactly that of the dis- 
turbed planet's own period of revolution round the Sun. 
Our exterior sentinels might fail thus to give us warn- 
ing of the foreign body because of being at the time in 
the opposite parts of their orbits. We should then be 
first apprised of its coming by Saturn, which would 
give us less prefatory notice. 



BIRTH OF A SOLAR SYSTEM 29 

It would be some twenty-seven years from the time 
it entered the range of vision of our present telescopes 
before it rose to that of the unarmed eye. It would then 
have reached forty-nine astronomical units' distance, 
or two-thirds as far again as Neptune. From here, 
however, its approach would be more rapid. Humanity 
by this time would have been made acquainted with 
its sinister intent from astronomic calculation, and 
would watch its slow gaining in conspicuousness with 
ever growing alarm. During the next three years it 
would have ominously increased to a first magnitude star, 
and two years and three months more have reached the 
distance of Jupiter and surpassed by far in lustre 
Venus at her brightest. 

Meanwhile the disturbance occasioned not simply in 
the outer planets but in our own Earth would have 
become very alarming indeed. The seasons would have 
been already greatly changed, and the year itself length- 
ened, and all these changes fraught with danger to every- 
thing upon the Earth's face would momentarily grow 
worse. In one hundred and forty-five days from the 
time it passed the distance of Jupiter it would reach 
the distance of the Earth. Coming from Vega, it would 
not hit the Earth or any of the outer planets, as the Sun's 
way is inclined to the planetary planes by some sixty 
degrees, but the effects would be none the less marked 
for that. Day and night alone of our astronomic re- 



30 THE EVOLUTION OF WORLDS 

lations would remain. It would be like going mad and 
yet remaining conscious of the fact. Instead of follow- 
ing the Sun we should now in whole or part, according 
to the direction of its approach, obey the stranger. For 
nineteen more days this frightful chaos would continue; 
as like some comet glorified a thousand fold the tramp 
dropped silently upon the Sun. Toward the close of 
the nineteenth day the catastrophe would occur, and 
almost in merciful deliverance from the already chaotic 
cataclysm and the yet greater horror of its contempla- 
tion, we should know no more. 

Unless the universe is otherwise articulated than we 
have reason to suppose, such a catastrophe sometime 
seems certain. But we may bear ourselves with equa- 
nimity in its prospect for two mitigating details. One is 
that there is no sign whatever at the moment that any 
such stranger is near. The unaccounted-for errors in the 
planetary theories are not such as point to the advent of 
any tramp. Another is, that judged by any scale of 
time we know, the chance of such occurrence is im- 
measurably remote. Not only may each of us rest 
content in the thought that he will die from causes of 
his own choosing or neglect, but the Earth herself will 
cease to be a possible abode of life, and even the Sun 
will have become cold and dark and dead so long be- 
fore that day arrives that when the final shock shall 
come, it will be quite ready for another resurrection. 



CHAPTER II 

EVIDENCE OF THE INITIAL CATASTROPHE IN OUR OWN 

CASE 

BY quite another class of dark bodies than those we 
contemplated in the last chapter is the immediate 
space about us tenanted. For that, too, is anything 
but the void our senses give us to understand. Could 
we rise a hundred miles above the Earth's surface we 
should be highly sorry we came, for we should incon- 
tinently be killed by flying brickbats. Instead of 
masses of a sunlike size we should have to do with bits 
of matter on the average smaller than ourselves but 
hardly on that account innocuous, as they would strike 
us with fifteen hundred times the speed of an express 
train. Only in one respect are the two classes of 
erratics alike, both remain invisible till they are upon 
us. Even so, the cause of their visibility is different. 
The one is announced by the light it reflects, the other 
by the glow it gives out on its destruction. These last 
are the meteorites or shooting-stars. They are as well 
known to every one for their commonness as, fortunately, 
the first are rare. On any starlight night one need not 
tarry long before one of these visitants darts across 

31 



32 THE EVOLUTION OF WORLDS 

the sky, a brilliant thread of fire gone almost ere it be 
descried. 

Usually this is all of which one is made aware. 
Silent, ghostlike, the apparition comes and goes, and 
nothing more of it is either seen or heard. But some- 
times there is a good deal more. Occasionally a large 
ball of flame shoots through the air, a detonation like 
distant thunder startles the ear, and a luminous train, 
persisting for several seconds, floats slowly away. 
Finally if one be fortunate to be near, — but not too 
near, — one or more masses of stone are seen to fall 
swiftly and bury themselves in the ground. These are 
meteorites : far wanderers come at last to rest in graves 
they have dug themselves. 

A great revolution has taken place lately in our ideas 
concerning meteorites. Indeed, it was not so very long 
ago, since modern man admitted their astronomic 
character at all. He looked as askance at them as he 
did at fossils. It was the fall at Aigle, in Switzerland, 
April 26, 1803, that first opened men's eyes to the fact 
that such falls actually occurred. It is more than a nine 
days' wonder at times how long men, as well as puppies, 
can remain blind. To admit that stones fell from 
heaven, however, was not to see whence they came. 
Their paternity was imputed to nearly every body in the 
sky. They were at first supposed to have been ejected 
from earthly volcanic vents, then from volcanoes in 



THE INITIAL CATASTROPHE 33 

the Moon. That they are of domestic manufacture is, 
however, negatived by the paths they severally pursue. 
Nor can they for like reason have been ejected from the 
Sun. 

The Earth v^as not their birthplace. It is alien 
ground in vs^hich they lie at last and from w^hich we 
transfer them to glass cases in our museums. This 
fact about their parentage they tell by the speed with 
which they enter our air. They become visible 100 
miles up and explode at from 20 to 10, and their speed 
has been found to be from 10 to 40 miles a second, 
which is that of cosmic bodies moving in large elliptic 
orbits about the Sun, — a speed greater than the Earth 
could ever have imparted. 

Four classes of such small celestial bodies tenant space 
where the planets move : sporadic shooting-stars, me- 
teorites, meteor-streams, and comets. The discovery 
of the relation of each of these to the solar system and 
then to each other forms one of the latest chapters of 
astronomic history. For they turn out to be generically 
one. 

It was long, however, before this was perceived. The 
first step was taken simultaneously by Professor Olm- 
stead of Yale and Twining in 1833 from reasoning on 
the superb November meteor-shower of that year. 
All the shooting-stars, "thick as snowflakes in a storm," 
had a common radiant from which they seemed to come. 

D 



34 THE EVOLUTION OF WORLDS 

Thus they argued that the meteors must all be travelling 
in parallel lines along an orbit which the previous shower, 
of 1799, showed to be periodic. This was the first rec- 
ognition of a meteor-swarm. 

The next advance was when Schiaparelli, in 1862, 
pointed out the remarkable connection between meteor- 
swarms and comets. On calculation the August meteor- 
stream and the comet of 1862 proved to be pursuing 
exactly the same path. Soon other instances of like 
association were discovered, and we now know mathe- 
matically that meteor-streams can be, deductively that 
they must be, and observationally that they are, dis- 
integrated comets. More than one comet has even 
been seen to split. 

Then came the recognition that comets are not 
visitors from space, as Sir Isaac Newton and Laplace 
supposed, but part and parcel of our own solar system. 
Without going into the history of the subject, which 
includes Gauss, Schiaparelli, and finally Fabry's great 
Memoir, much too little known, the proof can, I think, 
be made comprehensible without too much technique, 
thanks to the fact that the Sun is speeding through space 
at the rate of eleven miles a second. 

Orbits described by bodies under the action of a cen- 
tral force are always conic sections, as Sir Isaac Newton 
proved. There are two classes of such curves : those 
which return into themselves, such as the circle and 



THE INITIAL CATASTROPHE 35 

ellipse, and those which do not, the hyperbolae. If a 
body travel in the first or closed class about the Sun, it 
is clearly a member of his family; if in the second, it is 
a visitor who bows to him only in passing and never 
returns. Which orbit it shall pursue depends at a 
given distance solely upon the speed of the body; if 
that speed be one the Sun can control, the body will 
move in an ellipse; if greater, in an hyperbola. Ob- 
viously the Sun can control just the speed he can 
impart. Now a comet entering the system from with- 
out would already possess a motion of its own which, 
when compounded with the solar-acquired speed, 
would make one greater than the Sun could master. 
Comets, therefore, if visitors from space, should all 
move in hyperbolae. None for certain do; and only 
six out of four hundred even hint at it. Comets, then, 
are all members of the solar family, excentric ones, 
but not to be denied recognition of kinship for such 
behavior. 

Still, admittance to the solar family circle was denied 
to meteorites and shooting-stars. Thus Professor Kirk- 
wood, in 1 861, had considered '^that the motions of 
some luminous meteors (or cometoids, as perhaps they 
might be called) have been decidedly indicative of an 
origin beyond the limits of the solar system." Here 
cometoid was an apt coinage, but when comets were 
later shown not to be of extra-solar origin, the reasoning 



36 THE EVOLUTION OF WORLDS 

carried luminous meteors in its train. ''' Finally Schia- 
parelli, in 1 871, concluded an able Memoir on the sub- 
ject with the decision that " a stellar origin for meteorites 
was the most likely and that meteorites were identifiable 
with shooting-stars." f A pregnant remark this, though 
not exactly as the author thought, for instead of 
proving both interstellar, as he intended, both have 
proved to be solar bound. 

It was Professor Newton, in 1889, who first showed 
that meteorites were pursuing, as a rule, small elliptic 
orbits about the Sun, and that their motion was direct. 
He, too, was the first to surmise that meteorites are but 
bigger shooting-stars. 

Now, as to their connection. Of direct evidence we 
have little. A few meteors have been observed to come 
from the known radiants of shooting-stars. Two in- 
stances we have of the fall of meteorites during star 
showers. One in 1095, when the Saxon Chronicle tells 
us stars fell "so thickly that no man could count them, 
one of which struck the ground and when a bystander 
cast water upon it steam was raised with a great noise 
of boiling.'' The second case was the fall of a sider- 
ite, eight pounds' worth of nickel-iron, at Mazapil 
during the Andromede shower of 1885, which was by 

* " Mem. del Reale Inst. Lombardo," Vol. XII. Ill della 
serie III. 

f Quoted in " Luminous Meteors," Committee's Report for 
1870-1871, p. 48. 



THE INITIAL CATASTROPHE i^ 

many supposed to be a part of the lost Biela comet. 
It contained graphite enough to pencil its own history, 
but unfortunately could not write. The direction from 



IHp& 



The Radiant of a Meteoric Shower, showing also the Paths of Three 
Meteors which do not belong to this Shower — after Denning. 

which it came was not recorded, and so the connection 
between it and the comet not made out. 

If our direct knowledge is thus scanty, reasoning 
affords surer ground for belief. For at this point there 
steps in a bit of news about the family relations of 
shooting-stars from a source hardly to have been antici- 
pated. Indeed, it arose from the thought to examine a 
qualitative statement in Young's " Astronomy " quanti- 
tatively. Mathematics is simply precise reasoning. 



38 THE EVOLUTION OF WORLDS 

applied usually to the discovery that a pet theory will 
not work. But sometimes it presents one with an 
unexpected find. This is what it did here. 

It is an interesting fact of observation that more 
meteors are visible at six o'clock in the morning than at 
six o'clock at night in the proportion of 3 to i. This 



SUNRISE ""^ SUNSET 

METEORS 
Diagram explaining their proportionate visibility. 

denotes true paths. 

" apparent paths. 

" Harth's path. 

seeming preference for early rising is due to no matuti- 
nality on the part of the meteors, but to the matin aspect 
then presented by the Earth combined with its orbital 
motion round the Sun, For at six in the morning the 
observer stands on the advancing side of the Earth, 
at the bow of the airship ; at six at night he is at the 
stern. He, therefore, runs into the meteors at sunrise 
and slips away from them at sunset. He is pelted in the 
morning in consequence. Just as a pedestrian facing 
a storm gets wetter in front than behind. 

So far the books. Now let us examine this quanti- 
tatively according to the direction in which the meteors 



THE INITIAL CATASTROPHE 39 

themselves may be moving before the encounter. Sup- 
pose, in the first place, that they w^ere travelling in every 
possible direction, w^ith the average velocity of the most 
erratic members of the family, the great comets. On 
this supposition calculation shoves that we ought to 
meet 5.8 times as many at six in the morning as at six 
at night. If their orbits were smaller than this, say, 
something like those of the asteroids, we should find 7.6 
to I for the ratio. 

Suppose, however, that they were all travelling in the 
same sense as the Earth, direct as it is called in con- 
tradistinction to retrograde, and let us calculate what 
proportion in that case we should meet at the two 
hours respectively. It turns out to be 2.4 to i for the 
parabolic ones, 3.3 to i for the smaller orbited, or al- 
most precisely what observation shows to be the case.^* 
Here, then, a bit of abstract reasoning has apprized us of 
a most interesting family fact; to wit, that the great 
majority of shooting-stars are travelling in the same 
orderly sense as ourselves. Furthermore, as some must 
be moving in smaller orbits than the mean, others must 
be journeying in greater; or, in other words, shooting- 
stars are scattered throughout the system. In short, 
these little bodies are tiny planets themselves, as truly 
planets as the asteroids, — asteroids of a general instead 
of a localized habit. 

* Numerals refer to notes at end of book. 



40 THE EVOLUTION OF WORLDS 

Thus meteorites and shooting-stars are kin, and from 
the fact that they are pursuing orbits not very unhke 
our own we get our initial hint of a community of origin. 
Indeed, they are the httle bricks out of which the whole 
structure of our solar system was built up. What we 
encounter to-day are the left-over fragments of what 
once was, the fraction that has not as yet been swept 
up by the larger bodies. And this is why these latter- 
day survivors move, as a rule, direct. To run counter 
to the consensus of trend is to be subjected to greater 
chance of extermination. Those that did so have 
already been weeded out. 

From the behavior of meteorites we proceed to scan 
their appearance. And here we notice some further 
telltale facts about them. Their conduct informed us 
of their relationship, their character bespeaks their 
parentage. 

Most meteorites are stones, but one or two per cent 
are nearly pure iron mixed with nickel. When picked 
up, they are usually covered with a glossy thin black 
crust. This overcoat they have put on in coming 
through our air. Air-begotten, too, are the holes with 
which many of them are pitted. For entering our at- 
mosphere with their speed in space is equivalent to 
immersing them suddenly in a blowpipe flame of sev- 
eral thousand degrees Fahrenheit. Thus their sur- 
face is burnt and fused to a cinder. Yet in spite of 



THE INITIAL CATASTROPHE 



41 




being warm to the touch their hearts are still cosmically 
cold. The Dhurmsala meteorite falling into moist 
earth was found an hour afterwards coated with frost. 
Agassiz likened it to the 
Chinese culinary chef 
d' oeuvre "fried ice." It 
is the cold of space, 200° 
or more Centigrade be- 
low zero, that they bear 
within, proof of their 
cosmic habitat. 

That they are bits of 
a once larger mass is 
evident on their face. 
Their shape shows that 
they are not wholes but 
parts, while their con- 
stitution bespeaks them 
anything but elementary. 
Diagnosis of it yields 
perhaps their most interesting bit of news. For it shows 
their origin. Their autopsy proves them to contain 
thirty known elements, and not one that is new. The 
list includes all the substances most common on the 
Earth's surface, which is suggestive; but, what is still 
more instructive, these are combined into minerals 
which largely differ from those with which we are super- 




The Mart Iron. 
{Proc. Wash. Acad. ofSci. vol. II. plate VI.) 



42 



THE EVOLUTION OF WORLDS 



iicially familiar. Professor Newton, whose specialty 
they were, has said: "In general they show no resem- 
blance in their mechanical or mineralogical structure 
to the granitic and surface rocks of the Earth. One 




Section of Meteorite showing Widmannstattian Lines. 
(Field Columbian Museum, Chicago.) 

condition was certainly necessary in their formation, 
viz. the absence of free oxygen and of enough water to 
oxidize the iron." Thus they are not of the Earth 
earthy; nor yet, poor little waifs, of the upper crust of 
any other body. 

In them prove to be occluded gases, which can be got 
out by heating in the laboratory, and which must have 
got in when the meteorites were still subjected to great 
heat and pressure. For only thus could these gases 
have been absorbed. Both such heat and such pressure 
accuse some great solid body as origin of this flotsam 
of the sky. Fragments now, they owe to its disruption 



THE INITIAL CATASTROPHE 



43 



their present separate state. This parent mass must 
have been much larger and more massive than the 
Earth, as the great amount of occluded hydrogen, 
sometimes one- 
third the vol- 
ume at 500° C, 
of the meteorite 
seems to testify. 
The t w^o 
classes of mete- 
orites, the stone 
and the iron, 
show this fur- 
ther by the very 
differences they exhibit betw^een themselves. For both 
the amount and the proportions of the occluded gases 
in the two prove to be quite distinct. In the stones the 
quantity of gas is greater and the composition is di- 
verse. In the stones carbonic acid gas is common, 
carbon monoxide rare; in the irons the ratio is just the 
other way. Thus Wright found in nine specimens of 
the iron meteorites : — 

H 

54.1 % 




Meteorite, Toluca. 
(Field Columbian Museum, Chicago.) 



CO. 



CO 

11.5% 32.4% 

in ten of stone: — 
CO2 CO 

60.1% 3.4% 



CH4 

00 % of the total ; 



H 

32.0 % 



CH4 

2.1 % 



44 THE EVOLUTION OF WORLDS 

The stones are much Hghter than the iron, their 
specific gravities being as 3 to 7 or 8 for the metalhc. 
The stones, therefore, came from a more superficial 
layer of the body torn apart than the iron, and the 
composition of their occluded gases bears this out. 
Those in the stones are such as we may conceive ab- 
sorbed nearer the surface, those in the iron from re- 
gions deeper down. 

Here, then, the meteorites tell us of another, an 
earlier, stage of our solar system's history, one that 
mounts back to before even the nebula arose to which 
we owe our birth. For the large body to whose dis- 
memberment the meteorites were due can have been 
no other than the one whose cataclysmic shattering 
produced that very nebula which was for us the origin 
of things. The meteorites, by continuing unchanged, 
link the present to that far-off past. And they tell us, 
too, that this body must have been dark. For solid, 
they inform us, it was, and solidity in a heavenly body 
means deficiency of light. 

That such corroborative testimony to a cataclysmic 
origin is forthcoming in the sky we shall see by turning 
again to the spiral nebulae. 

Of the two classes of nebulae which we contemplated 
in the last chapter, the amorphous and the structural, 
there is more to be said than we touched on then. 

Not only in look are the two quite unlike, but the 



THE INITIAL CATASTROPHE 



45 



spectroscope shows that the difference in appearance 
is associated with dissimilarity of character. For the 
spectrum of the amorphous proves to consist of a few 
bright lines, due to hydrogen and nebulium chiefly, in 




Nebula Ijl V. 14 Cygni — after Roberts. 

the green, whence the name green nebulae. That of 
the spirals, on the other hand, is continuous, and there- 
fore white. The great nebula in Andromeda was one 
of the first in which this was recognized ; and the per- 
ception was pregnant, for no nebula defies resolution 
more determinedly than it. We may, therefore, infer 



46 



THE EVOLUTION OF WORLDS 



that it is not made up of stars, certainly big enough for 
us to see. On the other hand, from the fact that its 
spectrum is continuous it must be sohd or Hquid. 
Young pointed out that this did not follow, because 
a gas under great pressure also gives a continuous 




Nebula N. G. C. 1499 Persei — after Roberts. 

spectrum. But he forgot that here no such pressure 
could exist. A nebula of compressed gas could not 
have an irregular form and would have, in the case of 
the Andromeda nebula, a mass so enormous as to pre- 
clude supposition. Continuity of spectrum here means 
discontinuity of mass. The spectral solidity of the 
nebula speaks of a status quo ante, not of a condition 
of condensation now going on. 



THE INITIAL CATASTROPHE 



47 



Advanced spectroscopic means reveals that the 
spectra of these ^^ white" nebulae are not simply con- 



■ ■•:V;:'.-.'...Sv, A ■'.':;.;:. 

•■ • ". If- ■ .• "> . •' •.* -. ...'■.'•■•. ■ 

■ ■•■■ ■ • *.; . .f ••'..^•■•4 «•■ •• •. -".-■ 7 .y 
''' . • • " . . . t .v.'K.-. ',: '.[■■, 










■■■- *^':"t'f^%^- ^-.x^-'ntV. .-':!... -v: 

^- --•^•- vv^ ' :.; . ;^-: .VV- :..c t:;^^ ,. 
.i ■■%'. -•••.;.-■:■ .• .-•:.- ; ." . ■ .-■-. - •..-•'• 

-:--V^■.:•.:•^-^..--V■v■;V ,_...--■-. 
•:-,.• - .■ -•• . V.'. ->■.■,?;■ ■•-•--..•.: .'.♦ 



Nebula N. G. C. 6960 in Cygnus — after Ritchey. 

tinuous. Thus that of the Andromeda nebula shows 
very faint dark lines crossing it, apparently accordant 
with those of the solar spectrum and faint bright ones 
falling near and probably coincident with those of the 



48 THE EVOLUTION OF WORLDS 

Wolf-Rayet stars, due to hydrogen, helium, and so 
forth. These later observations make practically cer- 




Nebula M. 51 Canum Venaticorum — after Ritchey. 

tain what earlier ones permitted us just now only to in- 
fer : that it is not composed of stars, but of something 
subtler still; to wit, of meteorites. The reasoning is 
interesting, as showing that if one have hold of a true 
idea, the stars in their courses fight for him. 

Although Lockyer has long been of opinion that the 



THE INITIAL CATASTROPHE 49 

nebulae are composed of meteorites, the present 
argument differs from his. The way in which their 
spectra estabhsh their constitution may be outHned as 
follows : the white nebulae are from their structure 
evidently in process of evolution, and if they are in stable 
motion, as we suppose them to be, their parts are mov- 
ing round their common centre of gravity. As the 
white nebulae resist resolution as obstinately as the green, 
these parts must be not only solid but comminuted 
(composed of small particles). Now this would be the 
case were they flocks of meteorites such as we have seen 
composed our own system once upon a time. Though 
all are travelling round the centre of gravity of the 
flock, each is pursuing its own orbit slightly diff'erent 
from, and intersecting those of, its neighbors. Colli- 
sions between the meteors must therefore constantly 
occur, and the question is, are these shocks sufficient to 
cause light. Let us take our own system and consider 
two meteorites at our distance from the Sun, travelling 
in the same sense, the one in an ellipse, the other in a 
circle, with a major axis five per cent greater and meet- 
ing the other at aphelion. This would be no improper 
jostle for such heavenly bodies. If we calculate the 
speeds of both and deduct the elliptic from the circular, 
we shall have the relative speed of collision. It proves 
to be a half a mile a second or 30 times the speed of an 
express train. As such a train brought up suddenly 

E 



50 THE EVOLUTION OF WORLDS 

against a stone wall would certainly elicit sparks, we see 
that a speed 30 times as great, whose energy is 900 times 
greater, is quite competent to a shock sufficient to 
make us see stars en masse. But, indeed, there must be 
collisions much more violent than this; both because 
the central mass is often much greater and because the 
orbits differ much more, and the effect would increase 
as the square of the speed. The heat thus generated 
would cause the meteorites to glow, and at the same 
time raise the temperature of the gases in and about 
them. Furthermore, the light would come to us through 
other non-affected portions of gas between us and the 
scene of the collision. Thus all three peculiarities of 
the spectra stand explained : we have a continuous back- 
ground of light due to heated solid meteorites, the bright 
lines of glowing gases, and dark lines due to other gases 
not ignited, lying in our line of sight. 

In addition we should perceive another result. Col- 
lisions would be both more numerous and more pro- 
nounced toward the centre of the nebula, for it must 
speedily grow denser toward its core owing to the fall- 
ing in of meteorites, in consequence of shock. Being 
denser in the centre, the particles would there be thicker 
and be travelling at greater speed. The nebulae, there- 
fore, should be brightest at their centres, which is ac- 
cordant with observation. 

Thus from having offered themselves exemplars of the 



THE INITIAL CATASTROPHE 51 

way in which our own system came into being, the white 
nebulae assert their present constitution to be that from 
which we know our system sprang. 

Another suggestive fact about the present members 
of our solar system which has something to say about 
a past collision is the densities of the different planets. 
The average density of the four inner planets, Mars, 
the Earth, Venus, and Mercury is nearly four times 
that of the four outer ones Neptune, Uranus, Saturn, 
and Jupiter.^ The discrepancy is striking and cannot 
be explained by size, as the smallest are the most mas- 
sive, and if all were primally of like constitution, should 
be the least compressed. Nor can it be explained simply 
by greater heat tending to expand them, for Neptune 
and Uranus show no signs of being very hot. The 
minor differences between members of each group are 
probably explicable in part by these two factors, mass 
and heat, but the great gulf between the two groups 
cannot so be spanned. We are then driven to the sup- 
position that the materials composing the outer ones 
were originally lighter. Now this is precisely what 
should happen had all eight been formed by disruption 
of a previous body. For its cuticle would be its least 
dense portion, and on disruption would travel farthest 
away, not because of being lighter, but because of 
being on the outside. Parts coming from deeper down 
would remain near, and be denser intrinsically. 



52 THE EVOLUTION OF WORLDS 

What the present densities of the planets enable us 
to infer of the cataclysm from which they came, a re- 
markable set of spectrograms taken not long ago by 
Dr. V. M. Slipher, at Flagstaff, seems to confirm. 

The spectrograms in question were made possible by 
his production of a new kind of plate. His object was 
to obtain one which should combine sufficient speed 
with great photographic extension of the spectrum into 
the red. For it is in the red end that the absorption 
lines due to the planets' atmospheres chiefly lie. With 
the plates heretofore used it was impossible to go much 
beyond the yellow, the C line marking the Ultima 
Thule of attent. Not only was it advisable to get more 
particularity in the parts previously explored, but it was 
imperative to go beyond into parts as yet unknown. 
After several attempts he succeeded, the plates when 
exposed showing the spectra beyond even the A band. 
Of their wealth of depiction it is only necessary to say 
that in the spectrum of Neptune 130 lines and bands can 
easily be counted between the wave-lengths 4600 /x/x, 
7600 /x/x. Of these 31 belong to the planet, which 
compares with 6 found by Huggins, 10 by Vogel, and 
9 by Keeler in the part of its spectrum they were able 
to obtain. 

The result was a revelation. The plates exposed 
a host of lines never previously seen; lines that do not 
appear in the spectrum of the Sun, nor yet in the added 



h 


i^ 


;0 




Ld 


K~ 


n 






Ol 


an 


V 




SJ 






UI 


V 


T 


« 


h- 


^1 




V 




.^ 




'^ 



o 

o 



UJ 

t 

CL 



00 



< 



Q-. 




THE INITIAL CATASTROPHE 53 

spectrum of the atmosphere of the Earth, but are due 
to the planets' own envelopes. But this was only the 
starting-point of their disclosures. When in this man- 
ner he had taken the color signatures of Jupiter, Saturn, 
Uranus, and Neptune, an orderly sequence in their 
respective absorption bands stood strikingly confessed. 
In other words, their atmospheres proved not only 
peculiar to themselves and unlike what we have on 
Earth, but progressively so according to a definite law. 
That law was distance from the Sun. When the spectra 
were, arranged vertically in ordered orbital relation 
outward from the Sun, with that of the lunar for com- 
parison on top, r. surprising progression showed down 
the column in the strange bands, an increase in number 
and a progressive deepening in tint. The lunar, of 
course, gives us the Sun and our own air. All else must 
therefore be of the individual planet's own. Beginning, 
then, with Jupiter, we note, besides the reenforcement of 
what we know to be the great water-vapor bands * «,' sev- 
eral new ones, which show still darker in the spectrum of 
Saturn. The strongest of these is apparently not identi- 
fiable with a band in the spectra of Mira Ceti in spite of 
falling near it. Passing on to Uranus, we perceive these 
bands still more accentuated, and with them others, 
some strangers, some solar lines enhanced. Thus the 
hydrogen lines stand out as in the Sirian stars. All 
deepen in Neptune, while further newcomers appear. 



54 THE EVOLUTION OF WORLDS 

Thus we are sure that free hydrogen exists in large 
quantities in the atmospheres of the two outermost 
planets and most so in the one farthest off. Helium, 
too, apparently is there, and other gases which in part 
may be those of long-period stars, decadent suns, in 
part substances we do not know. 

From the fact that these bands are not present in the 
Sun and apparently in no type of stars, we may perhaps 
infer that the substances occasioning them are not ele- 
ments but compounds to us unknown. And from the 
fact that free hydrogen exists there alongside of them, 
and apparently helium, too, we may further conclude 
that they are of a lighter order than can be retained by 
the Earth. 

But now, we may ask, why should these lighter gases 
be found where they are ? It cannot be in consequence 
simply of the kinetic theory of gases from which a corol- 
lary shows that the heaviest bodies would retain their 
gases longest, because the strange gases are not appor- 
tioned according to the sizes of their hosts. Jupiter, 
by all odds the biggest in mass, has the least, and 
Saturn, the next weightiest, the next in amount. Nor 
can title to such gaseous ownership be lodged in the 
planet's present state. For though Jupiter is the 
hottest and Saturn the next so, the increased mass more 
than makes up in restraint what increased temperature 
adds in molecular volatility — as we perceive in the 
cases of the Sun and Earth. 



THE INITIAL CATASTROPHE 55 

No; their envelopes are increasingly strange because 
their internal constituents are different, and as hydro- 
gen is most abundant in Neptune, the lightest of all 
the gases, it is inferable that this planet's material is 
hghter. As distance from the Sun determines their 
atmospheric clothing, so distance decides upon their 
bodies, too. It was all a case of primogeniture. The 
light strange matter that constitutes them was so be- 
cause it came from the outer part of the dismembered 
parent orb. Neptune the outermost, Uranus the next, 
then Saturn and Jupiter came in that order from the 
several successive layers of the pristine body, while 
the inner planets came from parts of it deeper down. 
The major planets were of the skin of the dismembered 
body, we of its lower flesh. 

Very interesting the study of these curious spectral 
lines from the outer planets for themselves alone; 
even more so for what one would hardly have imagined : 
that they should actually tell us something of the genesis 
of our whole solar system. They corroborate in so far 
what the meteorites have to say. 

That the meteorites are solid and, except for their 
experiences in coming through our air, bear no marks of 
external heat, is a fact which is itself significant. It 
seems to hint not at a crash as their occasioning but 
at disruptive tidal strains. The parent body appears 
to have been torn apart without much development of 



56 THE EVOLUTION OF WORLDS 

heat. Perhaps, then, we had no gloriously pyrotechnic 
birth, but a more modest coming into existence. But 
about this we must ourselves modestly be content to 
remain for the present in the dark. 

Not the least important feature of the theory I have 
thus outlined is that it finishes out the round of evolu- 
tion. It becomes a conception sapiens in se ipso totus, 
teres atque rotundus. To frame a theory that carries 
one back into the past, to leave one there hung up in 
heaven, is for inconclusiveness as bad as the ancient 
fabulous support of the world, which Atlas carried 
standing on an elephant upheld by a tortoise. What 
supported the tortoise we were not told. So here, if 
meteorites were our occasioning, we must account for the 
meteorites, starting from our present state. This the 
present presentation does. 

Thus do the stones that fall from the sky inform us 
of two historic events in our solar system's career. They 
tell us first and directly of a nebula made up of them, out 
of which the several planets were by agglomeration 
formed and of which material they are the last un- 
gathered remains. And then they speak to us more 
remotely but with no less certainty of a time antedating 
that nebula itself, a time when the nebula's constitu- 
ents still lay enfolded in the womb of a former Sun. 

Man's interest in them hitherto has been, as with 
other things, chiefly proprietary. Greed of them has 



THE INITIAL CATASTROPHE 57 

grown so keen that legal questions have been raised of 
the ownership of their finding, and our courts have 
solemnly declared them not "wild game" but "real 
estate," and as such belonging to the owner of the land 
on which they fall. 

But to the scientific eye their estate is something 
more than "real," for theirs is the oldest real estate in 
the solar system. They were what they are now when 
the Earth we pride ourselves in owning was but a 
molten mass. 

So that when in future you see these strange stones 
in rows upon a museum's shelves, regard them not as 
rarities, in which each museum strives to outdo its 
neighbors by the quantity it can possess, but as rosetta 
stones telling us of an epoch in cosmic history long since 
passed away — of which they alone hold the key. 
Look at them as the literary do their books, for that 
which they contain, not as the bibliophile to whom a 
misprint copy outvalues a corrected one and by whom 
"uncuts" are the most prized of all. 



CHAPTER III 



THE INNER PLANETS 



WHEN we recall that the Ptolemaic system of the 
universe was once taught side by side with the 
Copernican at Harvard and at Yale, we are impressed, 
not so much with the age of our universities, as with 
the youth of modern astronomy and with the extraordi- 
nary vitality of old ideas. That the Ptolemaic system 
in its fundamental principle was antiquated at the 
start, the older Greeks having had juster concep- 
tions, does not lessen our wonder at its tenacity. But 
the fact helps us to understand why so much fossil 
error holds its ground in many astronomic text-books 
to-day. That stale intellectual bread is deemed better 
for the digestion of the young, is one reason why it 
often seems to them so dry. 

Before entering upon the problem of the genesis 
and career of a world, it is essential to have acquaint- 
ance with the data upon which our deductions are 
to rest. To set forth, therefore, what is known of the 
several planets of our solar system, is a necessary 
preliminary to any understanding of how they came 
to be or whither they are tending; and as our knowl- 

58 



THE INNER PLANETS 



59 



edge has been vitally affected by modern discoveries 
about them, it is imperative that this exposition of 
the facts should be as near as possible abreast of the 
research itself. I shall, therefore, give the reader 




Orbits of the Inner Planets. 



in this chapter a bird's-eye view of the present state 
of planetary astronomy, which he will find almost a 
different part of speech from what it was thirty years 
ago. It is not so much in our knowledge of their 
paths as of their persons that our acquaintance with 



6o THE EVOLUTION OF WORLDS 

the planets has been improved. And this knowledge 
it is which has made possible our study of their evo- 
lution as worlds. 

Could we get a cosmic view of the solar system by 
leaving the world we live on for some suitable vantage- 
point in space, two attributes of it would impose them- 
selves upon us — the general symmetry of the whole, 
and the impressively graded proportions of its par- 
ticular parts. 

Round a great central globular mass, the Sun, far 
exceeding in size any of his attendants, circle a series 
of bodies at distances from him quite vast, compared 
with their dimensions. These, his principal planets, 
are in their turn centres to satellite systems of like 
character, but on a correspondingly reduced scale. 
All of them travel substantially in one plane, a fact 
giving the system thus seen in its entirety a remark- 
ably level appearance, as of an ideal surface passing 
through the centre of the Sun. Departing somewhat 
from this general uniformity in their directions of 
motion, and also deviating more from circularity in 
their paths, some much smaller bodies, a certain dis- 
tance out, dart now up now down across it at different 
angles and from all the points of the compass, agree- 
ing with the others only in having the centre of the 
Sun their seemingly never attained goal of endeavor. 
These bodies are the asteroids. Surrounding the 



THE INNER PLANETS 6i 

whole, and even penetrating within its orderly pre- 
cincts, a third class would be visible which might be 
described for size as cosmic dust, and for display as 
heavenly pyrotechnics. Coming from all parts of 
space indifferently they would seem to seek the Sun 
in almost straight lines, bow to him in circuit, and 
then depart whence they came. For in such long ellip- 
ses do they journey that these seem to be parabolas. 
These visitants are the comets and their associates the 
meteor streams. 

Although for purposes of discrimination we have 
labelled the several classes apart, an essential fact about 
the whole company is to be noted : that no hard 
and fast line can be drawn separating the several 
constituents from one another. In size the members 
of the one class merge insensibly into the other. Some 
of the planets are hardly larger than some of the satel- 
lites; some of the satellites than some of the asteroids; 
some of the asteroids than comets and shooting stars. 
In path, too, we find every gradation from almost 
perfect circularity like the orbits of lo and Europa 
to the very threshold of where one step more would 
cease to leave the body a member of the Sun's family 
by turning its ellipse into an hyperbola. Finally, in 
inclination we have every angle of departure from or- 
thodox platitude to unconforming uprightness. This 
point, that heavenly bodies, like terrestrial ones, show 



62 THE EVOLUTION OF WORLDS 

all possible grades of indistinction, is kin to that spe- 
cific generalization by which Darwin revolutionized 
zoology a generation ago. It is as fundamental to 
planets as to plants. For it shows that the whole solar 
system is evolutionarily one. 

A second point to be noticed in passing is that 
undue inclination and excessive eccentricity go to- 
gether. The bodies that have their paths least circu- 
lar have them, as a rule, the most atilt. And with 
these two qualities goes lack of size. It is the smallest 
bodies that deviate most from the general consensus 
of the system. With so much by way of generic 
preface, the pregnancy of which will become apparent 
as we proceed, we come now to particular considera- 
tion of its members in turn. 

Nearest to the Sun of all the planets comes Mer- 
cury. So close is he to that luminary, and so far within 
the orbit of the earth, that he is not a very common 
object to the unaided eye. Copernicus is said never 
to have seen him, owing, doubtless, to the mists of the 
Vistula. By knowing when to look, however, he 
may be seen for a few days early in the spring in the 
west after sunset, or before sunrise in the east in au- 
tumn. He is then conspicuous, being about as bright 
as Capella, for which star or Arcturus he is easily 
mistaken by one not familiar with the constellations. 

His mean distance from the Sun is thirty-six million 



THE INNER PLANETS 63 

miles, but so eccentric is his orbit, the most so of any of 
the principal planets, that he is at times half as far 
off again as at others. Even his orbital behavior is 
the least understood of any in the solar system. His 
orbit swings round at a rate which so far has defied 
analysis. It may be a case of reflected perturbation, 
one, that is, of which the indirect effect from another 
body becomes more perceptible than would be the 
direct effect on the body itself. As yet it baffles 
geometers. 

As to his person, our ignorance until lately was 
profound. It is only recently that such fundamental 
facts about him as his size, his mass, and his density 
have been reached with any approach to precision. 
This was because he so closely hugs the Sun that 
observations upon his full, or nearly full, disk had 
never been attempted. When I say that his volume 
was not known to within a third of its amount, his 
mass not closer than one-half, while his received 
density was nearly double what we now have reason 
to suppose the fact, some idea of the depth of our nes- 
cience may be imagined. This, of course, did not pre- 
vent text-books from confidently misinstructing youth, 
or Nautical Almanacs from misguiding computers with 
figures that thus almost achieved immortality, so long 
had they passed current in spite of lacking that per- 
fection which is usually assigned as its warrant. 



64 



THE EVOLUTION OF WORLDS 



Schiaparelli first put astronomy on the right track. 
By attempting dayHght observations of the planet, not 



STI) 



ifji ^ 




yORD 



Sulla Rotazione di Mercurio — Di G. V. Schiaparelli. 

toward night, but actually at midday, he made some 
remarkable discoveries, and though he did not detect 
the hitherto erroneous values of the volume, the mass, 
or the density, his method of observation paved the 
way for their ascertainment. What he sought, and 
found, was evidence of markings upon the disk by 



THE INNER PLANETS 65 

which the planet's time of rotation might be deter- 
mined. Up to then, Schroeter's value of about twenty- 
four hours had been accepted, on very slender evidence 
indeed, and passed into all the books. But when the 
planet came to be observed by noon, very definite 
markings stood out on its face, which showed its rota- 
tion to take place, not in twenty-four hours, but in eighty- 
eight days. By a persistence equal to his able choice 
of observing time, he established this beyond dispute. 
He proved the revolutionizing fact that Mercury's pe- 
riods of rotation and of revolution were the same. 

He detected, too, the evidence in the position of the 
markings of the planet's great libratory swing due to 
the eccentricity of its orbit, a result as remarkable as 
a feat of observation as it was conclusive as a proof. 

If Schiaparelli had never done any other astronom- 
ical work, this study of Mercury would have placed 
him as the first observer of his day. For the observa- 
tions are so difficult that the planet not only baffled 
all his predecessors, but has foiled many since who 
are credited with being observers of eminence. 

In 1896 the study of Mercury was taken up at the 
Lowell Observatory in Arizona along the same lines 
that had proved so successful with Schiaparelli, but 
without using his observations as guide. Indeed, his 
papers had not then been read there. The two con- 
clusions were, therefore, independent of one another. 



66 THE EVOLUTION OF WORLDS 

The outcome was a complete corroboration and an 
extension of Schiaparelli's work. We shall begin , 
with the consideration of the most fundamental point. 
In the clear and steady air of Flagstaff, permitting of 
measurement of his disk up to within a few degrees 
of the Sun, Mercury was found to be much larger 
than previously thought. 

Instead of a diameter of three thousand miles he 
proved to have one of thirty-four hundred, making 
his volume nearly half as large again as had been 
credited him. These measures bore intrinsic evi- 
dence of their trustworthiness in an interesting man- 
ner, and at the same time produced internal testi- 
mony that accounted for the smallness of previous 
determinations. Measures heretofore had been made, 
usually if not invariably, either when the planet 
transited the Sun or when it exhibited a pronounced 
phase. Now in both these cases the planet looks 
smaller than it is. In the first case this is due to 
irradiation, the surrounding disk of the Sun encroach- 
ing both to the eye and to the camera upon the sil- 
houette of Mercury. And this inevitable effect had 
not been allowed for in the measures. In the second 
case the horns of the planet never seem to extend 
quite to their true position. This was rendered evi- 
dent by the Flagstaff series of measures, which began 
when the planet was a half-moon and continued till it 



THE INNER PLANETS 67 

was almost full. As it did so, the values for the diam- 
eter steadily increased, even after irradiation was 
allowed for, although this against the brilliant back- 
ground of the noonday sky must have been exceed- 
ing small, and tended in part to be diminished as the 
planet attained the full, because of its consequent near- 
ing of the Sun. The measures thus explained them- 
selves and vouched for their own accuracy.* 

Then came a curious bit of unexpected proof to 
corroborate them. In his '' Astronomical Constants," f 
published but a short time before, Newcomb had 
detected a systematic error in the right ascensions of 
Mercury which he was not able to explain. By dili- 
gent mousing that eminent computer had discovered 
that Mercury was registered by observers too far from 
the Sun on whichever side of him it happened to be, 
and in proportion roughly not to its distance off but 
to the phase the planet exhibited. When the disk was 
a crescent the discrepancy between observation and 
theory was large, and thence decreased as the planet 
passed to the full. He suspected the cause, and would 
have found it had he not considered the diametral 
measures of the planet too well assured to permit of 
doubt. As it was, he neglected a factor which has 

* New Observations of the Planet Mercury, Memoirs Amer. 
Acad. 1897. Vol. XII, No. 4. 

I " Astronomical Constants," 1895, pp. 67, 68. 



68 THE EVOLUTION OF WORLDS 

vitiated almost all the observations made on the planets 
up to v^ithin a fev^ years, the correction for irradia- 
tion. This w^as the case here. The received meas- 
ures, beginning with Bradley and ending with Todd, 
had almost without exception been made in transit, 
and, as no regard had been paid to the contracting 
effect of irradiation, had been invalidated in conse- 
quence. The new method supplied almost exactly the 
amount needed to explain the right ascensions, a sec- 
ond of arc, and in precise accordance with the place 
which the discrepancy demanded. 

About the mass there has been, and still is, great 
uncertainty. This is because it can only be found 
from the perturbing effect it has on Venus, the Earth, 
or Encke's comet. Modern determinations, however, 
are smaller than the older ones; thus Backlund in 
1894 got from the effect on Encke's comet only one- 
half the mass that Encke had, fifty-three years before. 
Probably the most reliable information comes from 
Venus, which Tisserand found to give for Mercury 
TTWFoo" ^^ ^^^ mass of the Sun, or 2X ^^ ^^^ mass of 
the Earth. If we take Too^io"oo' ^^ ^^^ nearest round 
number, we find the planet's density to be 0.66 that 
of the Earth. 

The same observations that disclosed at Flagstaff 
the planet's size revealed a set of markings on his face 
so definite as to make the rotation period unmistakable. 



THE INNER PLANETS 



69 



It takes place, as Schiaparelli found, in eighty-eight 
days, or the tinie of the planet's revolution round the 
Sun. The markings disclosed the fact, as Schiaparelli 
had also discovered, in a most interesting manner, for 




the ellipticity of the planet's orbit stood reflected in 
the sv^ing of the markings across the face of the disk, 
a definiteness in the proof of a really surprising kind. 
What this means we shall see in a subsequent chap- 
ter when we take up the mechanical problem of the 
tides. Another result that issued from the positions 
of the markings was the determination of the planet's 
pole. Except for the libration above noticed, the 



70 THE EVOLUTION OF WORLDS 

markings kept an invariable longitudinal position 
upon the illuminated disk, showing that the planet 
turned always the same face to the Sun; but latitudi- 
nally a difference was noticeable between their place in 
October-November, 1896, and in February-March, 
1897, the latter being 4° farther north. Now this 
is just what the orbital position should have caused, 
if the pole stood vertically to it. Thus a difference 
of 4° from perpendicularity should have been dis- 
cernible, had it existed, — a very small amount in 
such a determination. We may, therefore, conclude 
that the axis stands plumb to the orbit, and this is 
what theory demands. 

The state of things this introduces to us upon that 
other world is to our ideas exceeding strange. It is 
not so much the slowness of the diurnal spin, eighty- 
eight times as long as our own, which is surprising, as 
the fact that this makes its day infinite in length. 
Two antipodal hemispheres divide the planet, the one 
of which frizzles under eternal sun, the other freezes 
amid everlasting night. The Sun docs not, indeed, 
stand stock-still in the sky, but nods like some huge 
pendulum to and fro along a parallel of latitude. In 
consequence of libration the two great domains of 
day and night are sundered by a strip of debatable 
ground 23^° in breadth on either side, upon which the 
Sun alternately rises and sets. Here there is a true 



THE INNER PLANETS 71 

day, eighty-eight of our days in length from one sun- 
rise to the next. But its day and night are not ap- 
portioned ahke. The eastern strip has its dayhght 
briefer than its starhght hours; the western has them 
longer. Nor are different portions of the strips simi- 
larly circumstanced in their sunward regard. Only 
the edge next perpetual day has anything approach- 
ing an equal distribution of sunlight and shade. The 
farther one just peeps at the Sun for a moment every 
eighty-eight days, and then sinks back again into ob- 
scurity. 

The transition from day to night is equally instan- 
taneous and profound. For little or no twilight here 
prolongs the light; since the air, if there be any at all, 
is too thin to bend it to service round the edge to 
illuminate the night. When the Hbratory Sun sets, 
darkness like a mantle falls swiftly over the face of 
the ground. No evidence of atmosphere has ever 
been perceived, and theory informs that it should be 
nearly, if not wholly, absent. 

In consequence of the rigid uprightness of the 
planet's axis, seasons do not exist. Their nearest 
simulacrum comes from the seeming dilatation of the 
Sun during half the year, and its apparent contraction 
during the other half. It expands so much between 
its January and its July as to receive more heat in the 
ratio of nine to four. A seasonless, dayless, and almost 



72 THE EVOLUTION OF WORLDS 

yearless planet, it is better to look at than to look 
from; but its study opens our eyes to the great diver- 
sity which even one of our nearest neighbors exhibits 
from what we take as matters of course on Earth. 

That what we take offhand to be purely astronomic 
phenomena should turn out to be so essentially of the 
particular world, worldly, clarifies vision of what 
these really are, and how dependent on and interwoven 
with everyday life astronomy is. Or, we may consider 
it turned about and realize how purely astronomic 
relations, such abstract mechanical matters as rota- 
tions and revolutions, result in completely changing 
the very face and character of the globe concerned. 
Mercury to-day stares forever at the Sun. The 
markings we see have stereotyped this stare to its in- 
evitable result. For they seem to mark a globe sun- 
cracked. At such a condition the curious crisscross 
of dark, irregular lines certainly hints, accentuated 
and perfected as it is by a bounding curve where the 
mean sunward side terminates to the enclosing them 
as by the carapace of a tortoise. Though they can- 
not probably be actual cracks, however much they 
may resemble such, yet they may well owe their exist- 
ence to that fundamental cause. 

In color the planet is ghastly white; of that wan 
hue that suggests a body from which all life has fled. 
Far whiter than Venus in point of fact, the rosy tint 



THE INNER PLANETS j^ 

with which it sparkles in the sunset glow is all bor- 
rowed of the dying day and vanishes when the planet 
is looked at in the uncompromising light of noon. 
Seen close together once at Flagstaff it was possible 
directly to compare the two; when Mercury, although 
lit by the Sun two and a half times as brilliantly as 
Venus, was, surface for surface, more than twice as 
faint. Miiller has found its intrinsic brightness about 
that of our Moon, which in some respects it resembles, 
though it apparently departs widely from any simi- 
larity in others. The bleached bones of a world ; that 
is what Mercury seems to be. / 

Venus comes next in order outward from the Sun. 
To us her incomparable beauty is partly the result of 
propinquity: nearness to ourselves and nearness to 
the Sun. Relatively so close is she to both that she 
does not need the Sun's withdrawal to appear, but 
may nearly always be seen in the daytime in clear air 
if one knows where to look for her. Situate about 
seven-tenths of our own distance from our common 
giver of light and heat, she gets about double the 
amount that falls to our lot, so that her surface is pro- 
portionately brilliantly illuminated. Being also rela- 
tively near us, she displays a correspondingly large 
surface. 

But though part of her lustre is due to her position, 
a part is her own. Direct visual observation, as we 



74 THE EVOLUTION OF WORLDS 

remarked above, shows her intrinsic brightness to be 
more than five times that of Mercury, square mile to 
square mile of surface for the two. Now this has 
been determined very carefully photometrically by 
Miiller at Potsdam. The result of his inquiry was to 
indicate that Mercury shines with 0.17 of absolute 
reflection, Venus with 0.92. So high a value has 
seemed to many astronomers impossible, because so 
far surpassing that which has tacitly been taken as 
the ne plus ultra of planetary brightness, that of cloud, 
0.72. 

Now, one of the direct outcomes of the study of 
Venus at the Lowell Observatory was an explanation 
of this seemingly incredible phenomenon. When the 
planet came to be critically examined there under 
conditions of seeing which permitted discovery, mark- 
ings very faint, but nevertheless assurable, stood pre- 
sented on the planet's face. These markings, of which 
we shall have more to say in a moment, had this of 
pertinency to our present point, that they kept an in- 
variable position to one another. They thus betrayed 
themselves to be surface features. Furthermore, their 
dimness was as invariable an attribute of them as their 
place. They were not obscured on some occasions 
and revealed at others, but stayed, so far as one might 
judge, permanently the same. They were thus neither 
clouds themselves nor subject to the caprice of cloud. 



THE INNER PLANETS 75 

The old idea that Venus was a cloud-wrapped planet 
and owed her splendor to this envelope, vanished liter- 
ally into thin air. 

It is precisely because she is not cloud-covered that 
her lustre is so great. She "clothes herself with light 
as with a garment" by a physical process of some 
interest. As becomes the Mother of the Loves, this is 
gauze of the most attenuated character, and yet a 
wonderful heightener of effect. For it consists solely 
of the atmosphere that compasses her about. It is 
well known that a substance when comminuted re- 
flects much more light than when condensed into a 
solid state. Now an atmosphere is itself such a com- 
minuted affair, and, furthermore, holds in suspension a 
variety of dust. This would particularly be the case 
with the atmosphere of Venus, as we shall have reason 
to see when we consider the conditions upon that 
planet made evident by study of its surface markings. 
To her atmosphere, then, she owes four-fifths or more 
of her brilliancy. And this stands corroborated by 
the low albedo of both Mercury and the Moon, which 
have no atmosphere, and by the intermediate lustre of 
Mars, which has some, but little.^ 

The rotation time of Venus, the determination, that 
is, of the planet's day, is one of the fundamental astro- 
nomical acquisitions of recent years. For upon it 

* A sir. Nach. No. 3406. Monthly Notices R. A. S., March, 1897. 



76 THE EVOLUTION OF WORLDS 

turns our whole knowledge of the planet's physical 
condition. More than this, it adds something which 
must be reckoned with in the framing of any cos- 
mogony. It is not a question of academic accuracy 
merely, of a little more or a little less in actual 
duration, but one which carries in its train a com- 
pletely new outlook on Venus and sheds a valuable 
sidelight upon the history of our whole planetary 
system. 

Unconsciously influenced, one is inclined to think, 
by terrestrial analogies, astronomers for more than a 
couple of centuries, ever since the time of the first 
Cassini in 1666, deemed the day of Venus to be just 
under twenty-four hours in length. So well attested 
was its determination, and so precisely figured to the 
minute, that it imposed itself upon text-books which 
stated it as an acquired fact down to the last second. 
Nevertheless, Schiaparelli was not so sure, and pro- 
ceeded to look into the matter. He first looked for 
himself, and then looked up all the old observations. 
His chief observational departure was observing by 
day as .near to noon as possible; because then the 
planet was highest, to say nothing of the taking off 
from its glare by the more brilliant sky. From certain 
dark markings around two bright spots near the 
southern cusp, of one of which spots the detection 
dates from the time of Schroeter, and from a long. 



THE INNER PLANETS '^^ 

dark streak stretching thence well down the disk, he 
convinced himself that no such period as twenty-four 
hours could possibly be correct, inasmuch as whenever 
he looked, the markings were always there. His notes 
read, "Same appearance as yesterday,'^ day after day, 
until he would really have saved ink and penmanship 
had he had the phrase cut into a die and stamped. 
He concluded that the rotation was at least six months 
long, and was probably synchronous with the planet's 
time of revolution. This was in 1889. In 1895 he 
became still more sure, and showed how the older 
observations were really compatible with what he had 
found. 

In 1896 the subject was taken up at Flagstaff. Very 
soon it became evident there that markings existed 
on the disk, most noticeable as fingerlike streaks 
pointing in from the terminator, faint but unmistakable 
from the identity of their successive presentaxion. 
Schroeter's projection near the south cusp was also 
clearly discernible as well as two others, one in mid- 
terminator, one near the northern cusp. Schiaparelli's 
dark markings also came out, developing into a sort 
of collar round the southern pole. Other spots and 
streaks also were discernible, and all proved permanent 
in place. By watching them assiduously it was pos- 
sible to note that no change in position occurred in 
them, first through an interval of five hours, then 



78 THE EVOLUTION OF WORLDS 

through one of days, then of weeks. Care was taken 
to guard against illusion. It thus became evident 




Venus. October, 1896-MARCH, 1897 — Drawings by Dr. Lowell. 

that they bore always the same relation to the illumi- 
nated portion of the disk. This illuminated part, 



THE INNER PLANETS 79 

then, never changed. In other words, the planet 
turned always the same face to the Sun. The fact 
lay beyond a doubt, though of course not beyond a 
doubter.* 

The years that have passed since these observations 
were made have brou2;ht corroboration of them. Sev- 




Venus. April 12, 1909, 3H 26M-4H 22M — by Dr. Lowell. 

eral observers at Flagstaff have seen and drawn them 
and added discoveries of their own, among whom 
are especially to be mentioned, of the observatory 
staff: Miss Leonard, Dr. Slipher, and Mr. E. C. 
Slipher.f 

In character these markings were peculiar and dis- 
tinctive. In addition to some of more ordinary char- 

* Monthly notices R. A. S., March, 1897. 
■j" Lowell Observatory Bulletin 6. 



8o THE EVOLUTION OF WORLDS 

acter were a set of spokelike streaks which started 
from the planet's periphery and ran inwards to a 
point not very distant from the centre. The spokes 
started well-defined and broad at the edge, dwindling 
and growing fainter as they proceeded, requiring the 
best of definition for their following to their central 
hub. 

The peculiar symmetry thus displayed, a symrhetry 
associated with the planet's sunrise and sunset line, 
or, strictly speaking, what would be such did the Sun 
for Venus ever rise or set, would seem inexplicable, 
except for that very association. When we reflect, 
however, upon what this means, a very potent cause 
for them becomes apparent, so potent that surprise is 
turned into appreciation that nothing else could well 
exist. That Venus turns on her axis in the same 
time that she revolves about the Sun, in consequence 
of which she turns always the same face to him, must 
cause a state of things of which we can form but faint 
conception, from any earthly analogy. One face 
baked for countless aeons, and still baking, backed by 
one chilled by everlasting night, while both are still 
surrounded by air, must produce indraughts from 
the cold to the hot side of tremendous power. A 
funnel-like rise must take place in the centre of the 
illuminated hemisphere, and the partial vacuum thus 
formed would be filled by air drawn from its periphery, 



THE INNER PLANETS 



8i 



which, in its turn, would draw from the regions 
of the night side. Such winds would sweep the sur= 




i ^~""^^-vN. " 




1 ^^ ''' 

1 \ 


■f - 


i k 


^^ 




// . 



North 




South 



P^ 



Venus. 



to .Mni 



Showing convec- 
tion, currents in the 
planet's atmosphere. 



II 

Showing shift in 
central barometric 
depression due to 
rotation of the planet 
afifecting the winds. 



face as they entered, becoming less superficial as they 
advanced, and the marks of their inrush might well 



82 THE EVOLUTION OF WORLDS 

be discernible even at the distance we are off. Deltas 
of such inroad would thus seam the bounding circle of 
light and shade. 

Another result of the aerial circulation would be 
the removal of all moisture from the sunward face, 
and its depositing in the form of ice upon the night 
one. For the heated air would be able to carry much 
water in suspension, which, on cooling, after it had 
reached the dark hemisphere would unload it there. 
In the low temperature there prevailing, this moisture 
would all be frozen, and so largely estopped from re- 
turn. This process continuing for ages would finally 
deplete one side of all its water to heap it up in the 
form of ice upon the other. 

Now it is not a little odd that a phenomenon has 
been observed upon Venus which seems to display 
just this state of things. Many observers have noted 
an ashen light on the dark side of her disk. Some 
have tried to account for it as Earth shine, the same 
earth-reflected light that makes dimly visible the old 
moon in the new moon's arms. But the Earth is too 
far away from Venus to permit of any such effect; 
nor is there any other body that could thus relieve its 
night. But if the night hemisphere of Venus be one 
vast polar sheet, we have there a substance able to 
mirror the stars to a ghostlike gleam which might be 
discernible even from our distant post. 



THE INNER PLANETS 83 

Thus when we reason upon them we see that the 
pecuHar markings of the planet lose their oddity, be- 
coming the very pattern and prototype of what we 
should expect to view. Interpreted, they present us 
the picture of a plight more pitiable even than that 
of Mercury. For the nearly perfect circularity of 
Venus' orbit prevents even that slight change from 
everlasting sameness which the libration of Mercury's 
affords. To Venus the Sun stands substantially stock- 
still in the sky, — a fact which must prove highly reas- 
suring to Ptolemaic astronomers there, if there be any 
still surviving from her past. No day, no seasons, 
practically no year, diversifies existence or records 
the flight of time. Monotony eternalized, — such is 
Venus' lot. 

What visual observations have thus discovered of 
the rotation time of Venus, with all that follows from 
it, the spectroscope at Flagstaff has confirmed. At 
Dr. Slipher's hands, spectrograms of the planet have 
told the same tale as the markings. It was with spe- 
cial reference to this point that the spectrograph there 
was constructed, and the first object to which it was 
directed was Venus.* 

The planet's rotation time was to be investigated 
by means of the motion it brought about in the line of 
sight. Visual observation, telescopically, reveals motion 

* Lowell Observatory Bulletin No. 3. 



84 THE EVOLUTION OF WORLDS 

thwart-wise by the displacement it produces in the field 
of view; spectroscopic observation discloses motion to 
or from the observer by the shift it causes in the spectral 
lines due to a stretching or shortening of their wave- 
lengths. 

The spectroscope is an instrument for analyzing 
light. Ordinary light consists of light of various 
wave-lengths. By means of a prism or grating these 
are dispersed into a colored ribbon or band, the longer 
waves lying at the red end of the spectrum, as the ribbon 
is called, the shorter at the violet. Now the spectro- 
scope is primarily such a prism or grating placed 
between the image and the observer, by means of which 
a series of colored images of the object are produced. 
In order that these may not overlap and so confuse 
one another, the light is allowed to enter the prism only 
through a narrow slit placed across the telescopic image 
of the object to be examined. Thus successive images 
of what is contained by the slit are presented arranged 
according to their wave-lengths. In practice the rays 
of light from the slit enter a small telescope called 
the collimator, and are there rendered parallel, in which 
condition they fall upon the prism. This spreads 
them out into the spectrum and another small tele- 
scope focusses them, each according to its kind, into a 
spectral image band which may then be viewed by 
the eye or caught upon a photographic plate. 



THE INNER PLANETS 85 

Now, if an object be coming toward the observer, 
emitting or reflecting light as it does so, each wave- 
length of its spectrum will be shortened in proportion 
to the relative speed of its approach as compared with 
the speed of light, because each new wave is given out 
by so much nearer the observer and in reflection the 
body may also meet it. Reversely it will be length- 
ened if the object be receding from the observer or 
he from it. This would change the color of the object 
were it not that while each hue moves into the place 
of the next, like the guests at Alice's tea-party in 
Wonderland, some red rays pass off the visible spec- 
trum, but new violet rays come up from the infra- 
violet and the spectrum is as complete as before. 
Fortunately, however, in all spectra are gaps where 
individual wave-lengths are absorbed or omitted, and 
these, the Hnes in the spectrum, tell the tale of shift. 
Now if a body be rotating, one side of it will be ap- 
proaching the observer, while the opposite side is 
receding from him, and if the slit be placed perpen- 
dicular to the axis about which the spin takes place, 
each spectral line will appear not straight across the 
spectrum of the object, but skewed, the approaching 
side being tilted to the violet end, the receding side 
to the red. 

This was to be the procedure adopted for the rotation 
of Venus. By placing the slit parallel to the ecliptic, 



86 THE EVOLUTION OF WORLDS 

or, more properly, to the orbit of Venus, which is prac- 
tically the same thing, it found itself along what we 
have reason to suppose the equator of the planet. 
Even a considerable error on this point would make 
little difference in the rotational result. In order that 
there might be no question of illusion or personal 
bias, photographs instead of eye observations of the 
spectrum were made. For reference and check side 
by side with that of Venus were taken on either hand 
the spectra of iron, made by sparking a tube containing 
the vapor of that metal. The vapor, of course, had no 
motion with regard to the observer, and therefore its 
spectral lines could have no tilt, but must represent 
motional verticality. 

Dr. Slipher chose his time astutely. He selected 
the occasion when Venus was passing through supe- 
rior conjunction, or the point in her orbit as regards us 
directly beyond the sun. At first sight this might 
seem to be the worst as well as the most impracticable 
of epochs, inasmuch as the planet is then not only at 
her farthest from the Earth, but in a line with the Sun, 
and so drowned in his glare. But in point of fact 
any tilt of the spectral lines is then, owing to phase, 
twice what it is at elongation, and exceeds still more 
what it is when Venus has her greatest lustre.^ In his 
purpose he was abetted by the Flagstaff air, which 
enabled the planet to be spectrographed much nearer 



THE INNER PLANETS 87 

the sun than would otherwise have been the case. He 
thus selected the best possible opportunity. To guard 
against any subsequent bias on the part of the examiner 
of the plates, after the spectroscope had taken a plate it 
was then reversed, and the process repeated on another 
one, the iron being sparked as before. What had been 
the right side of Venus with regard to the red end 
of the spectrum thus became the left one, and vice 
versa. In this manner, when the plates came to be 
measured for tilt, the measurer would have no indi- 
cation from the spectrum itself which way the lines 
might be expected to tilt; he could, therefore, not 
be influenced either consciously or unconsciously in 
his decision. 

Eight plates with their comparison ferric spectra 
were thus secured; four with the spectroscope direct, 
four with it reversed. They were then shuffled, their 
numbers hidden, and given to Dr. Slipher to measure. 
The spectral lines told their own story, and without 
prompting. All the plates agreed within the margin 



mrntmmm »m>MMm*'m ■•«^* .»!»•> ■■ wtm*- »«« *iimmhim<*mm'>mmmm»m9'm ■mimtimmmm.mmm-**mm^ 



Spectrogram op Venus, showing its long day — V. M. Slipher, 
Lowell Observatory, 1903. 

of error accordant with their possible precision, a pre- 
cision some thirty times that of Belopolski's experiment 
on the same lines, — a result not derogatory of that 



88 THE EVOLUTION OF WORLDS 

investigator, but merely illustrative of superior equip- 
ment. They show^ed conclusively that a rotation of 
anything like twenty-four hours v^as out of the ques- 
tion. They yielded, indeed, testimony to a negative 
rotation of three months, w^hich, interpreted, means 
that so slow a spin as this was beyond their power 
to precise. 

For Dr. Slipher was at no less care to determine just 
what precision was possible in the case, although a 
speed corresponding to a spin of twenty-four hours on a 
globe the size of Venus is well known to be spectroscop- 
ically measurable. It would mean a motion toward us 
of one thousand miles an hour, or about a third of a mile 
a second. The tilt occasioned by this speed is well within 
the spectroscope's ability to disclose. Not content with 
this,however, by two special investigations, he proved the 
spectroscope's actual limits of performance to be far 
within the quantity concerned. One of them was the 
determination by the same means and in like manner of 
the rotation time of Mars, the length of that planet's 
day, which in other ways we know to the hundredth of 
a second, and which is 24^ 37"" 23^ 66. Now Mars offers 
a test nearly twice as difficult as Venus, even supposing 
the apparent disks of the two the same, because his diam- 
eter being less in the proportion roughly of one-half, 
the actual speed of a particle at his edge is less for the 
same time of rotation in the like proportion, and it is only 



THE INNER PLANETS 89 

with the speed in miles, not in angular amount, that the 
spectroscope is concerned. Nevertheless, when a like 
number of plates were tried on him, they indicated on 
measurement a rotation time within an hour of the true. 
This corresponds to half an hour on Venus. We see, 
therefore, that had Venus' day been anywhere in the 
neighborhood of twenty-four hours, Dr. Slipher's in- 
vestigation would have disclosed it to within thirty-one 
minutes. 

This result was further borne out by a similar test 
made by him of Jupiter. Inasmuch as the diameter of 




Spectrogram of Jupiter, giving the length of its day by the tilt of 
ITS spectral lines — V. M. Slipher, Lowell Observatory. 

Jupiter is twelve times that of Venus, while the rotation 
time is 9^' 5o'^.4 at the equator, the precision attained 
on Venus should here have been about a minute. And 
this is what resulted. Slipher found the rotation time 
spectrographically 9^' 50™, or in accordance with the 
known facts, while previous determinations with the 
spectroscope had somehow fallen short of it. 

The care at Flagstaff with which the possibility of 
error was sought to be excluded in this investigation of 
the length of Venus' day and the concordant precision 



90 THE EVOLUTION OF WORLDS 

in the results are worthy of notice. For it is by thus 
being particular and systematic that the accuracy of the 
determinations made there, in other lines besides this, 
has been secured. 

In size, Venus of all the planets most nearly ap- 
proaches the Earth. She is 7630 miles in diameter to 
the Earth's 7918. Her density, too, is but just inferior 
to ours. And she stands next us in place, closest in 
condition and constitution in the primal nebula. Yet 
in her present state she could hardly be more diverse. 
This shows us how dangerous it is to dogmatize upon 
what can or cannot be, and how enlightening beyond 
expectation often is prolonged and systematic study of 
the facts. 

The next planet outward is our own abode. It is one 
of which most of us thinkwe know considerable from ex- 
perience and yet about which we often reason cosmically 
so ill. If we knew more, we should not deem ourselves 
nearly so unique. For we really differ from other mem- 
bers of our system not more than they do from one an- 
other. Much that appears to us fundamental is not so 
in fact. Thus many things which seem matters of 
course are merely accidents of size and position. Our 
very day and night upon which turn the habits of all 
animals and, even in a measure, those of plants, are, as we 
have seen, not the possession of our nearest of cosmic kin. 
Our seasons which both vegetally and vitally mean so 



THE INNER PLANETS 91 

much are absent next door. And so the list of our 
globe's peculiar attributes might be run through to the 
finding of diversity to our familiar ways at every turn. 
But, as we shall see later, these differences from one 
planet to the next are not only not incompatible with a 
certain oneness of the whole, but actually help to make 
the family relationship discoverable. Analogy alone 
is a dangerous guide, but analogy crossed with diver- 
sity is of all clews the most pregnant of understanding. 
The very fact that we can tell them apart when we see 
them together, as the Irishman remarked of two brothers 
he was in the habit of confusing, points to their brotherly 
relation. 

Proceeding still further, we come to Mars at a mean 
distance of one hundred and forty-one million miles. 
Smaller than ourselves, his diameter is but a little over 
half the Earth's, or forty-two hundred miles, his mass 
one-ninth of ours, and his density about seven-tenths 
as much. Here, again, but in a different way, we 
find a planet unlike ourselves, and we know more about 
him than of any body outside the Earth and Moon. 
So much about him has been set forth elsewhere that it 
is enough to mention here that no oceans diversify his 
surface, no mountains relieve it, and but a thin air 
wraps it about, — an air containing water-vapor, but 
so clear that the surface itself is almost never veiled 
from view. 



92 THE EVOLUTION OF WORLDS 

About the satellites Mars possesses, Deimos and Pho- 
bos, we may perhaps say a word, as recent knowledge 
concerning them exemplifies the care now taken to such 
ascertainment and the importance of considering factors 
often overlooked. Soon after they were discovered in 
1877, they were measured photometrically, with the 
result of giving a diameter of six miles to Deimos and 
one of seven miles to Phobos, and these values un- 
challenged entered the text-books. When the satellites 
came to be critically considered at Flagstaff, it was 
found that these determinations were markedly in error, 
Phobos being very much the larger of the two, the actual 
values reaching nearer ten miles for Deimos and thirty- 
six for Phobos. 

In getting the Flagstaff values, the size to the eye 
of the satellite was corrected for the background upon 
which it shone; for the background is all-important to 
the brilliancy of a star. In the case of a small star near 
a planet, the swamping glare of the planet is something 
like the inverse cube of its distance away. Furthermore, 
the Flagstaff observations indicated how the previous 
error had crept in. For before correction for the dif- 
fering brilliancies of the field of view, the apparent 
size of the satellites judged by conspicuousness was 
about six to seven. The photometric values must have 
been taken just as they came out, no correction appar- 
ently having been made for the background. Now the 



THE INNER PLANETS 93 

background is a fundamental factor in all photometric 
determinations, a factor somewhat too important in this 
case to neglect, since it affected the result 2500 per 
cent. 



CHAPTER IV 

THE OUTER PLANETS 

BEYOND Mars lies the domain of the asteroids, a 
domain vast in extent, that, untenanted by any 
large planet, stretches out to Jupiter. Occupied solely 
by a host of little bodies agreeing only in lack of size, 
even this space seems too small to contain them, for 
recent research has shown some transgressing its 
bounds. One, Eros, discovered by De Witt, more 
than trenches on Mars' territory, having an orbit 
smaller than that of the god of war, and may be con- 
sidered perhaps the forerunner of more yet to be found 
between Mars and the Earth. On the other side, 
three recently detected by Max Wolf at Heidelberg 
have periods equal to that of Jupiter, and in their 
motions appear to exemplify an interesting case of celes- 
tial mechanics pointed out theoretically by Lagrange 
long before its corroboration in fact was so much as 
dreamt. Achilles, Patroclus, and Hector, as the triad 
are called, so move as always to keep their angular 
distance from Jupiter unaltered in their similar circuits 
of the Sun. 

Before considering these bodies individually, we may 

94 



THE OUTER PLANETS 



95 



"well look upon them en hloc, inasmuch as one attribute 
of the asteroids concerns them genetically rather than 
specifically, and is of great interest both from a mechani- 




Orbits of the Outer Planets. 

cal and an historical point of view. For, in fact, it is 
what led to their discovery. Titius of Wittenburg, 
about the middle of the eighteenth century, noticed a 
curious relation between the distances from the Sun of 
the then known planets. It consisted in a sort of regu- 
lar progression, but with one significant gap. Bode was 
so struck by the gap that he peopled it with a supposed 



96 THE EVOLUTION OF WORLDS 

planet, and so brought the relation into general regard 
in 1772. In consequence, it usually bears his name. 
It is this : if we take the geometrical series, 3, 6, 12, 24, 
48, 96 and add 4 to each term, we shall represent to a 
fair degree of precision the distances of the several 
planets, beginning with Mercury at 4 and ending with 
Saturn at 100, which was the outermost planet then 
known. All the terms were represented except 24 + 4, 
or 28 — a gap lying between Mars and Jupiter. When 
Uranus was discovered by Sir WilHam Herschel in 1781 
and was found to be travelling at what corresponded 
to the next outer term 192+4, or 196, the opinion be- 
came quite general that the series represented a real 
law and that 28 must be occupied by a planet. Von 
Zach actually calculated what he called its analogical 
elements, and finally got up in 1800 a company to look 
for it which he jocularly described as his celestial 
police. Considering that Bode's law is not a law at all, 
but a curious coincidence, as Gauss early showed in its 
lack of precision and in its failure to mark the place of 
Mercury with any approach to accuracy, and as the 
discovery of Neptune amply bore out, it was perhaps 
just in fate that the honor of filling the gap did not fall 
to any of the '* celestial police,'' but to an Italian 
astronomer, Piazzi, at the time engaged on a new star 
chart. An illness of Piazzi caused it to be lost almost 
as soon as found. In this plight an appeal was made 



THE OUTER PLANETS 97 

to the remarkable Gauss, just starting on his career. 
Gauss undertook the problem and devised formulae 
by which its place was predicted and the planet itself 
recovered. It proved to fit admirably the gap. But 
it had hardly been recovered before another planet 
turned up equally filling the conditions. Ceres, the 
first, lay at 26.67 astronomical units from the Sun; 
Pallas, the second, at 27.72. Two claimants were one 
too many. But the inventive genius of Olbers came 
to the rescue. By a bold hypothesis he suggested 
that since two had appeared where only one was 
v/anted, both must originally have formed parts of 
a single exploded planet. He predicted that others 
would be detected by watching the place where the 
explosion had occurred, to wit : where the orbits of 
Ceres and Pallas nearly intersected in the signs of the 
Virgin and the Whale. 

For in the case of an explosion the various parts, 
unless perturbed, must all return in time to the scene 
of the catastrophe. By following his precept, two more 
were in fact detected in the next two years, Juno and 
Vesta. His hypothesis seemed to be confirmed. 
No new planets were discovered, and the old fulfilled 
fairly what was required of them. Lagrange on cal- 
culation gave it his mathematical assent. 

Nevertheless, it was incorrect, as events eventually 
showed, though for forty years it slept in peace, no 



98 THE EVOLUTION OF WORLDS 

new asteroids being found. We now know that this 
was because the rest were all much smaller, and for 
such nobody looked. It was not till 1845 ^^^^ Hencke, 
an ex-postmaster of Driessen in Prussia, after fifteen 
years of search detected another, Astraea, of the nth 
magnitude. After this discoveries of them came on 
apace, until now more than six hundred are known, 
and their real number seems to be legion. But those 
discovered are smaller each year on the average, 
showing that the larger have already been found. 
Their orbits are such that they cannot possibly ever 
have all formed part of a pristine whole. The idea, 
not the body, was exploded. For they are now recog- 
nized as having always been much as they are to-day. 
They prove to be thickest at nearly the point where 
Bode's law required, the spot where Ceres and Pallas 
were found. The mean of their distances is less, being 
2.65 instead of 2.8 astronomical units, probably simply 
because the nearer ones are easier discovered. The 
fact that they are clustered most thickly just inside 
2.8 astronomical units implies that there of all points 
within the space between Mars and Jupiter a planet 
would have formed if it could. A definite reason 
exists for its failure to do so — Jupiter's disturbing 
presence. Throughout this whole region Jupiter's 
influence is great; so great that his scattering effect 
upon the particles exceeds their own tendency to come 




O 
05 



Hi 



THE OUTER PLANETS 99 

together. We see this in the arrangement of the 
orbits. If we plot the orbits of the asteroids, we shall 
be struck by the emergence of certain blanks in the 
ribbon representing sections of their path. It is the 
woof of a plaid of Jupiter's weaving. The gaps are 
where asteroids revolving about the Sun would have 
periods commensurate with his, f, ^, |^, y, and the like. 
Such bodies would return after a few revolutions, 
five of theirs, for instance, to Jupiter's two, into the 
same configurations with him at the same points of 
their orbits. Thus the same perturbation would be 
repeated over and over again until the asteroid's path 
was so changed that commensurability ceased to 
exist. And it would be long before perturbation 
brought it back again. Thus the orbits are constantly 
swinging out and in, all of them within certain limits, 
but those are most disturbed which synchronize with 
his. In this manner he has fashioned their arrange- 
ment and even prevented any large planet from form- 
ing in the gap. 

Such restrictive action is not only at work to-day 
in the distribution of the asteroids and in the partitions 
of Saturn's ring, but it must have operated still more 
in the past while the system was forming. To Pro- 
fessor Milham of Williamstow^n is due the brilliant 
suggestion that this was the force that fashioned the 
planetary orbits. For a planet once given off from a 



100 THE EVOLUTION OF WORLDS 

central mass would exercise a prohibitive action upon 
any planet trying to form within. In certain places 
it would not allow it to collect at all. The evolution 
of the solar family would resemble that of some human 
ones in which each child brings up the next in turn. 
So that the planetary system made itself, as regards 
position, a steadily accumulative set of prohibitions 
combining to leave only certain places tenantable. 

In this manner we may perhaps be brought back 
to Bode's law as representing within a certain degree 
of approximation a true mechanical result, although 
no such exact relation as the law demands exists. 
That a relation seemingly close to it is necessitated by 
the several successive inhibitions of each planet upon 
the next to form, is quite possible. 

One other general trait about their orbits is worth 
animadversion. In spite of being eccentric and in- 
clined, they are all traversed in the same sense. Every 
one of the asteroids travels direct like the larger planets. 
In this they differ from cometary paths, which are as 
often retrograde as direct. Thus in more ways than 
one they hold a mid-course in regularity between the 
steady, even character of the planets proper and what 
was for long deemed the erratic behavior of the 
cometary class of cosmic bodies. Very telling this 
fact will be found with regard to the genesis of the 
solar family, as we shall see later. 



THE OUTER PLANETS loi 

With regard now to their more individual characteris- 
tics, the asteroids may be said to agree in one point — 
their diversity, not only to all the larger members of the 
solar family, but to one another. For they travel in or- 
bits ranging in ellipticity all the way from such as nearly 
approach circles to ellipses of cometary eccentricity. , 
They voyage, too, without regard to the dynamical plane 
of the system, or, what is close to it, the ecliptic; depart- . 
ing from the general level often 30° and, in one instance, 
that of the little planet dubbed W. D., by as much as 
48°. This eccentricity and inclination put them in a 
class by themselves. It is associated and unquestionably 
connected mechanically with another trait which like- 
wise distinguishes them from the planets more particu- 
larly called — their diminutive size. Only four — Vesta, 
Ceres, Pallas, and Juno — out of the six hundred odd 
now known exceed a hundred miles in diameter, and the 
greater number are hardly over ten or twenty miles 
across. Very tiny worlds indeed they would seem, could 
we get near enough to them to discern their forms and 
features. Curiously enough, reasoning on certain light 
changes they exhibit has enabled us to divine something 
of their shapes, and even character. Thus it was soon 
perceived that Eros fluctuated in the light he sent 
us, being at times much brighter than at others. In 
February and March, 1901, the changes were such that 
their maximum exceeded three times their minimum two 



102 THE EVOLUTION OF WORLDS 

hours and a half later. Then in May the variation van- 
ished. More than one explanation has been put for- 
ward, but the best so far, because the most simple, is 
that the body is not a sphere but a jagged mass, a moun- 
tain alone in space, and that as it turns upon its axis 
first one corner and then another is presented to our 
view or throws a shade upon its neighbor. When the 
pole directly faces us, no great change occurs, especially 
if it also nearly faces the Sun. Yet even this fails to 
explain all its vagaries. 

Eros is not alone in thus exhibitingvariation. Sirona, 
Hertha, and Tercidina have also shown periodic vari- 
ability, and it is suspected in others. Indeed, it would 
be surprising did they not show change. For they are 
too small to have drawn their contents into symmetry, 
and so remain as they were when launched in space. 
Mammoth meteorites they undoubtedly are. 

With the asteroids we leave the inner half of the Sun's 
retinue and pass to the outer. Indeed, the asteroids 
not only mark in place the transition bound between 
the two, but stamp it such mechanically. In their 
own persons they witness that no large body was here 
allowed to form. The culmination of coalition was 
reached in Jupiter, and that very acme of accretion 
prevented through a long distance any other. 

In bulk, the major planets compared with the inner or 
terrestrial ones form a class apart; and amongthe major 



THE OUTER PLANETS 103 

Jupiter is by all odds first. His mass is 318 times the 
Earth's and his volume nearly 1400 times hers. From 
this it appears that his density is very much less. In- 
deed, his substance is only fractionally denser than water. 
This and its tremendous spin, carrying a point at its 
equator two hundred and eighty thousand miles round 
in less than ten hours, flatten it to a very marked 
oval with an ellipticity of ^575-. 
Not the least beautiful of the 
revelations of astronomy are 
the geometrical shapes of the 
heavenly bodies, proceeding 
from nearly perfect spheres 
like the Sun or Moon to 




'«-"l*!> <^ 





marked spheroids like Jupi- Drawing of Jupiter by Dr. 

Lowell. April 12, 1907. 

ter or Saturn. So enormous 

are the masses and the forces concerned that the forms 
assumed under them are mechanically regular. They 
are the visible expression of gravitation, and so delight 
the brain while they satisfy the eye. 

It is to appreciation of the detail visible on Jupiter's 
disk that modern advance in the study of the planet 
is indebted. Examination has shown its features 
to be of great interest. To Mr. Stanley Williams of 
Brighton, England, much of our knowledge is due, 
and Mr. Scriven Bolton has also made some interest- 
ing contributions. The big print of the subject, read 



104 THE EVOLUTION OF WORLDS 

long ago, is that the planet's disk is noticeably banded 
by dark belts. Two characteristics of these belts 
are important. One is that they exhibit a regular 
secular progression with the lapse of years, the 
south tropical belt being broader and more salient for 
many years in succession, and then gradually fading 
out while the northern one increases in prominence. 
It has been suspected that the rhythm of their change 
is connected with that of sun spots. The second is 
that the belts do not preserve in their several features 
the same relation in longitude toward one another. 
They all rotate, but at different speeds. There could 
be no better proof that Jupiter is no solid, but a seething 
mass of heavy vapors boiling like a caldron. Tem- 
pered by distance we can form but a faint idea of the 
turmoil there going on. Further indication of it is 
furnished by its glow. For all the dark belts are a 
beautiful cherry red, a tint extending even to the 
darkish hoods over the planet's caps. This hue 
comes out well in good seeing, and best, as with all 
planetary markings, in twilight, not at night, because 
the excessive brightness of the disk is then taken oflF, 
preventing the colors from being swamped. 

This brings us to the planet's albedo, which Miiller 
at Potsdam has found to be 75 per cent. Now the 
interest attaching to this determination is twofold, 
that it bespeaks cloud and that it seems to imply 



THE OUTER PLANETS 



105 



\ 



something else. The albedo of cloud is 72 per cent 

of absolute whiteness. What looks like cloud, then, 

is such, on that distant ^^..^r^^T^^-m^r-^ 

disk. But Jupiter sur- ,^ 

passes cloud in lustre, since *' ^ ^ 

his albedo exceeds 72 per 

cent. Yet a large part of 

his surface is strikingly 

darker than that. The in- x 

ference from this is that he j 

shines by intrinsic light, in Jupiter and its wisps.— a draw- 

iNG BY Dr. Lowell, April ii, 1907. 

part at least. The fact 

may not be stated dogmatically, as there is no astro- 
nomic determination so uncertain as this one of deter- 
mining albedoes, and therefore Herr Miiller's results 

must be accepted with every 
reserve, but they suggest 
that Jupiter is still a semi- 
sun, to be recognized as such 
by light as well as heat, 
though his self-luminosity, 
if it exist at all, can hardly 
exceed a dull red glow. 

A modern detection on 
Jupiter's disk has been that 
of wisps or lacings across the bright equatorial belt, 
a detail of importance due to Mr. Scriven Bolton. 




II. 

Jupiter and its wisps. — A draw 
ING BY Dr. Lowell, April ii, 1907 



io6 THE EVOLUTION OF WORLDS 

Requested to look for them, the observatory at Flag- 
staff was not long in corroborating this interesting 
phenomenon. The peculiarity about them pointed out 
by Mr. Bolton is that they traverse the belt at an angle 
of about 45° to the vertical, proceeding from caret- 
shaped dark spots projecting into the bright belt 
from the dark ones on either side. They exist all 
round the equator and are found indifferently dex- 
trous or sinister — sometimes vertical. For there are 
others that go straight across. Nor are they confined 
to the bright equatorial belt, but are to be seen travers- 
ing all of the bright belts both north or south up to 
the polar hoods. From its sombreness it seems that 
we are here regarding a phenomenon in the negative; 
remarking it by what it has left behind, not by what it 
has accomplished. For the wisps are not wisps of 
cloud, since they are dark, not light, but gaps strung 
out in the clouds themselves. 

Recently photographs of Jupiter have been secured 
at Flagstaff, by the new methods there of planetary 
photography, showing a surprising amount of detail. 
The wisps come out with certainty, and the white 
spots, which are such a curious feature of the disk, have 
also left their impress on the plate. Not the least of 
the services thus rendered by the camera is the accurate 
positioning of the belts made possible by it. Micro- 
metric measures are all very well when nothing better 



THE OUTER PLANETS 



10' 



is attainable, but any one who has made such upon a 
planet's disk swinging like a lantern in the field of 
view under a variety of causes instrumental and 
optical, knows how encumbered they inevitably are 
with error. To have the disk caught s. 

and fixed on a plate where it may be 
measured at leisure and as often as 
one likes, is a distinct advance toward 
fundamental accuracy. Measures thus 
effected upon the Jupiter images of 
1909 proved the bright equatorial belt 
to lie exactly upon the planet's equator 
when allowance was made for the tilt 
of the planet's axis toward the Earth. 
This showed that the aspect of the 
planet toward the Sun had no effect 
upon the position of the belt. Jupiter's 
cloud formation, therefore, is not depend- 
ent, as all ours are, upon the solar heat. 

A like indifference to solar action is exhibited in the 
utter obliviousness of the belts to day or night. To 
them darkness and light are nugatory alike. They re- 
appear round the sunrise edge of the disk just as they 
left it when they sank from sight round the sunset one, 
and they march across its sunlit face without so much 
as a flicker on their features. 

Yet this seeming immobility from moment to moment 




Photograph of 

Jupiter, 1909. 

P. L. 



io8 THE EVOLUTION OF WORLDS 

takes place in what is really a seething furnace, the 
fiery glow of which we catch below the vast ebullition of 
cloud in the cherry hue of its darker portions. Distance 
has merged the turmoil into the semblance of quiescence 
and left only its larger secular changes to show. Even 
so the Colorado River from the brinkof the Grand Caiion 
is seen apparently at rest, the billows of its rapids so 
stereotyped to stability one takes the rippled sand bank 
for the river and the billows of the river for the ripple 
marks of its banks. 

At twice the distance of Jupiter we cross the orbit 
of Saturn. Here the ringed planet, with an annual 
sweep of twenty-nine and a half of our years, pursues 
his majestic circuit of the Sun. Diademed with three 
or more circlets of light and diamonded by ten satel- 
lites, he rivals in his cortege that of his own lord. In 
some ways his surpasses the Sun's. For certainly his 
retinue is the more spectacular of the two; the more so 
that it is much of it fairly comprised within a single 
glance. Very impressive Saturn is as, attended thus, 
he sails into the field of view. 

In our survey we may best begin with his globe. If 
Jupiter's compression is striking, Saturn's is positively 
startling when well displayed. This happens but at rare 
intervals. As the plane of his equator is almost exactly 
that of the rings, the flattening is conspicuous only on 
those occasions when the rings disappear because their 




m 

15 
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Pi 
W 

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o 
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C/3 



THE OUTER PLANETS 109 

plane passes through the Hne of sight. Seen at such 
times the effect of the discrowned orb is so strange as to 
suggest delusion. This occurred two years ago in 1907, 
and when the planet was picked up by its position and 
entered the field unheralded by its distinctive append- 
age, it was almost impossible to believe there had not 
been some mistake and a caricatured Jupiter had 
taken its place. For the flattening outdoes that of 
Jupiter as 3 to 2, being ^ of the equatorial diameter. 
Such a bulging almost suggests disruption and is due 
to the extreme lightness of the planet's substance, 
which is actually only 0.72 of that of water. Like 
Jupiter, the disk exhibits belts, though very much 
fainter, and, like his, these are of a cherry red. As 
the planet's albedo is even greater, 0.78 of absolute 
whiteness, as deduced from H. Struve's measures of 
the diameter, the same suspicion of shining, at least in 
part, from inherent light, applies equally to him. But 
it is practically certain that in neither case does this 
light equal that of the planet's clouds, or add anything 
to them. Both planets are red-hot, not white-hot. 
The determination of the albedo depends upon that of 
the diameter, and an increase in the latter would lower 
the albedo to that of cloud. 

His most unique possession are his rings. Broad, yet 
tenuous, they weigh next to nothing, being, as Struve 
has dubbed them, ** Immaterial light." Nevertheless, 



no THE EVOLUTION OF WORLDS 

it is not their lightness but their make-up that prevents 
from lying uneasy the head that wears this crown. 

The mechanical marvel was not appreciated by early 
astronomers, who took it for granted that they were 
what they seemed, solid, flat rings, all of a piece. Even 
Laplace considered it sufllicient to divide them up con- 
centrically to insure stability. To Edouard Roche of 
Montpellier, as retiringly modest as he was penetrat- 
ingly profound, is due the mathematical detection that 
to subsist they must be composed of discrete particles, 
— brickbats. Clerk Maxwell called them, when, later, 
unaware of Roche's work, he proved independently 
the same thing in his essay on Saturn's rings. Peirce, 
too, in ignorance of Roche, had half taken the same 
step a little before, showing that they must at least be 
fluid. Then in 1895 Keeler ingeniously photographed 
the spectrum of both ball and rings to the revealing of 
velocities in the line of sight of the different portions of 
the spectrum exactly agreeing with the values me- 
chanics demanded. 

The rings have usually been considered to be flat. 
At the time of their disappearance, however, knots 
have been seen upon them. It is as if their filament 
had suddenly been strung with beads. At the last 
occurrence of the sort in 1907, these beads were partic- 
ularly well seen at several observatories, and were 
critically studied at Flagstaff'. In connection with a 



THE OUTER PLANETS iii 

new phenomenon detected there, that of a dark core 
in the shadow the rings threw across the planet's face, 
an explanation suggested itself to account for both 
them and it: to wit, that the rings were not really 
flat, but tores; rings, that is, like an anchor ring, any 
cross-section of which would be of the nature of an 
oval flattened on its inner side. The cogency of the 
explanation consisted in its solution not only of the 
appearances but of the cause competent to bring those 
appearances about. 

For measurement showed that the knots were per- 
manent in position, which, since the ring revolved, 
indicated that they extended all round it in spite of 
their not seeming to do so, and that their distances 
from Saturn were just what this cause should pro- 
duce. 

The action observed was a corollary from the 
important principle of commensurability of orbital 
period. As we saw in the case of the asteroids, if two 
bodies be travelling round a third and their respec- 
tive periods of revolution be commensurate, they will 
constantly meet one another in such a manner that 
great perturbation will ensue and the bodies be thrown 
out of commensurability of period. 

What has happened to the asteroids has likewise 
occurred in Saturn's rings. The disturber in this case 
has been, not Jupiter, as with them, but one or other 



112 THE EVOLUTION OF WORLDS 

of Saturn's own satellites. For when we calculate the 
problem, we find that Mimas, Enceladus, and Tethys 
have periods exactly commensurate with the divisions 
of the rings; in other words, these three inner satellites, 
whose action because of proximity is the greatest, 
have fashioned the rings into the three parts we know, 
called A, the outermost; B, the middle one; and C, the 
crepe ring, nearest to the body of the planet. Mimas 
has been the chief actor, though helped by the two 
others, while Enceladus has further subdivided ring A 
by what is known as Encke's division. 

Such has been the chief action of the satellites on the 
rings : it has made them into the system we see. But 
if we consider the matter, we shall realize that a second- 
ary result must have ensued — when we remember that 
the particles composing the rings must be very crowded 
for the rings to show as bright as they do, and also 
that, though relatively thin, the rings are nevertheless 
some eighty miles through. 

Now it is evident that any disturbance in so closely 
packed a system of small bodies as that constituting 
Saturn's rings must result in collisions between the 
bodies concerned. Particles pulled out or in must 
come in contact with others pursuing their own paths, 
and as at each collision some energy is lost by the 
blow, a general falling in toward the planet results. 
At the same time, as the blow will not usually be exactly 



THE OUTER PLANETS 



113 




in the plane in which either particle was previously 

moving, both will be thrown more or less out of the 

general plane of their fellows, and the ring at that 

point, even if originally flat, will not remain so. For 

the ring, though very 

narrow relatively, has 

a real thickness, quite 

sufficient for slantwise 

collision, if the bodies 

impinge. 

Now the knots or 
beads on the rings ap- 
peared exactly inside 
the points where the 
satellites' disturbing 
action is greatest, or, 
in other words, in pre- 
cisely their theoretic 
place. We can hardly doubt that such, then, was 
their origin.* 

The result must be gradually to force the particles as 
a rule nearer the planet, until they fall upon its surface, 
while a few are forced out to where they may coalesce 
into a satellite, — a result foreseen long ago by Maxwell. 
It is this process which in the knots we are actually 
witnessing take place, and which, like the corona about 

* Paper by the writer in the Phil. Mag., April, 1908. 



T3nt Tofie 
.3 C 

9v 



Norerntc \ ifo?. 



Torx. Tore 
C B 

9p - 



¥ 



114 THE EVOLUTION OF WORLDS 

the eclipsed Sun, only comes out to view when the 
obliterating brightness of the main body of the rings 
is withdrawn by their edgewise presentation. 

The reason the out-of-plane particles are m.ost 
numerous just inside the point of disturbance is not 
only that there the action throwing them out is most 
violent, but .that all the time a levelling action quite 
apart from disturbance is all the time tending to 
reduce them again to one plane, as we shall see further 
on when we come to the mechanical forces at work. 
Thus the tore is most pronounced on its outer edge, 
and falls to a uniform level at its inner boundary. 
The effect is somewhat as represented in the adjoining 
cut, in which the vertical scale is greatly magnified : — 




B 



The Tores of Saturn. Not drawn to scale. 

With Saturn ended the bounds of the solar system as 
known to the civiHzed world until 1781. On March 13 
of that year Sir William Herschel in one of his telescopic 
voyages through space came upon a strange object which 
he at once saw was not a star, because of its very percep- 
tible round disk, and which he therefore took for a pecul- 



THE OUTER PLANETS 115 

iar kind of comet. Nearly a year rolled by before Lexell 
showed by calculation of its motion that it was no comet, 
but undoubtedly a new planet beyond Saturn travel- 
ling at almost twice that body's mean distance from 
the Sun. 

Bj^ reckoning backward, it was found to have been 
seen and mapped several times as a star, — no less than 
twelve times by Lemonnier alone, — and yet its plan- 
etary character had slipped through his fingers. It can 
even be seen with the naked eye as a star of the 6th 
magnitude, and its course is said to have been watched 
by savage tribes in Polynesia long before Sir William 
Herschel discovered it. 

Its greenish blue disk indicates that it is about thirty- 
two thousand miles in diameter, and its mass that its 
density is about 0.22 of the Earth's or, like Jupiter's, 
somewhat greater than water. Of its surface we prob- 
ably see nothing. Indeed, it is very doubtful if it have 
any surface properly so called, being but a ball of va- 
pors. Its flattening, ^ according to Schiaparelli, which 
is probably the best determination, agrees with the den- 
sity given above, indicating its substance to be very 
light. Belts have faintly been descried traversing its 
disk after the analogy of Jupiter and Saturn. These 
would be much better known than they are but for the 
great tilt of the planet's axis to the ecliptic, so that during 
a part of its immense annual sweep its poles are pointed 



ii6 THE EVOLUTION OF WORLDS 

nearly at the Earth, and its tropical features, the places 
where the belts lie, are wholly hidden or greatly fore- 
shortened from our point of view. As the planet's year 
is eighty-four of our years long, it is only at intervals 
of forty odd years that the disk is well enough displayed 
to bring the belts into observable position. 

The planet is attended by four satellites, — Ariel, Um- 
briel, Titania, and Oberon, — a midsummer night's 
dream to a watcher of the skies. They travel in a plane 
inclined 98° to the ecliptic, so that their motion is nearly 
up and down to that plane and even a little backward. 
Whether their plane is also the equatorial plane of the 
planet, we do not know for certain. The observations 
as yet are not conclusive one way or the other. If the 
two planes should turn out not to coincide, it will open 
up some new fields in celestial mechanics. The belts 
have been thought to indicate divergence, but the most 
recent observations by Perrotin on them minimize this. 
They suggest, too, a rotation period of about ten hours, 
which is what we should expect. 

Its albedo, or intrinsic brightness, is, according to 
Miiller, 0.73, or almost exactly that of cloud. This 
tallies with the lack of pronouncement of the belts and 
is another argument against the reality of the recent 
diametral measurements, as all Miiller's values are 
got by dividing the amount of light received by the 
amount of surface sending it. If the diameter were 



THE OUTER PLANETS 117 

much less than thirty-two thousand miles, the result- 
ing albedo would become impossibly high. 

If we know but little about the actual surface of 
Uranus, we know now a good deal about its atmosphere. 
And this partly because atmosphere is almost all that it 
is. The satellites are the only solid thing in the system. 
If we needed a telltale that the solar system had evolved, 
the gaseous constitution of its primaries and the con- 
densed state of their attendants would sufficiently in- 
form us. Probably all the major planets are nothing 
but gas. It has been debated whether Jupiter be al- 
most all vapor with a solid kernel beneath, or vapor 
entirely. That he grows denser toward the core is 
doubtless the case, but that he is anywhere other than 
a gaseous fluid is very unlikely. For if he had really 
begun to condense, he must have contracted to far 
within his present dimensions. The same is true of 
Uranus. 

The surprising thing about Uranus is the enor- 
mous extent of his atmosphere. The earliest spectro- 
scopists perceived this, but the more spectroscopy 
advances, the greater and more interesting it proves 
to be. By pushing inquiry into the red end of the 
spectrum, hitherto a terra incognita, Dr. Slipher has 
uncovered a mass of as yet unexplained revelation. 
Of these remarkable spectrograms we shall speak 
later. Here it is sufficient to say that so great is the 



ii8 THE EVOLUTION OF WORLDS 

absorption in the red that only the blue and green in 
anything like their entirety get through; which ac- 
counts for the well-known sea-green look of the planet. 
Furthermore, the spectroscope shows that this atmos- 
phere, or the great bulk of it, must lie above what we 
see as the contour of the disk. For the spectroscope 
is as incapable of seeing through opacity as the eye, 
though it distances the eye in seeing the invisible. It 
is not what is condensed into cloud, but what is not, 
of which it reveals the presence. We are thus made 
aware of a great shell of air enveloping the planet. 

In Uranus, then, we see a body in an early amor- 
phous state, before the solid, the liquid, and the gaseous 
conditions of matter have become differentiate and 
settled each into distinctive place. Without even an 
embryo core its substance passes from viscosity to cloud. 

Neptune has proved a planet of surprises. Though 
its orbital revolution is performed direct, its rotation 
apparently takes place backward, in a plane tilted 
about 35° to its orbital course. Its satellite certainly 
travels in this retrograde manner. Then its appearance 
is unexpectedly bright, while its spectrum shows bands 
which as yet, for the most part, defy explanation, though 
they state positively the vast amount of its atmosphere 
and its very peculiar constitution. But first and not 
least of its surprises was its discovery, — a set of sur- 
prises, in fact. For after owing recognition to one of 



THE OUTER PLANETS 119 

the most brilliant mathematical triumphs, it turned 
out not to be the planet expected. 

** Neptune is much nearer the Sun than it ought to 
be," is the authoritative way in which a popular histo- 
rian puts the intruding planet in its place. For the 
planet failed to justify theory by not fulfilling Bode's 
law, which Leverrier and Adams, in pointing out the dis- 
turber of Uranus, assumed *' as they could do no other- 
wise." Though not strictly correct, as not only did 
both geometers do otherwise, but neither did otherwise 
enough, the quotation may serve to bring Bode's law 
into court, as it was at the bottom of one of the strangest 
and most generally misunderstood chapters in celestial 
mechanics. 

Very soon after Uranus was recognized as a planet, 
approximate ephemerides of its motion resulted in 
showing that it had several times previously been 
recorded as a fixed star. Bode himself discovered 
the first of these records, one by Mayer in 1756, and 
Bode and others found another made by Flamstead 
in 1690. These observations enabled an elliptic orbit 
to be calculated which satisfied them all. Subse- 
quently others were detected. Lemonnier discovered 
that he had himself not discovered it several times, 
cataloguing it as a fixed star. Flamstead was spared a 
like mortification by being dead. For both these ob- 
servers had recorded it two or more nights running. 



120 THE EVOLUTION OF WORLDS 

from which it would seem almost incredible not to 
have suspected its character from its change of place. 

Sixteen of these pre-discovery observations were found 
(there are now nineteen known), which with those 
made upon it since gave a series running back a hun- 
dred and thirty years, when Alexis Bouvard prepared 
his tables of the planet, the best up to that time, pub- 
lished in 1 82 1. In doing so, however, he stated that 
he had been unable to find any orbit which would 
satisfy both the new and the old observations. He 
therefore rejected the old as untrustworthy, forgetting 
that they had been satisfied thirty years before, and 
based his tables solely on the new, leaving it to pos- 
terity, he said, to decide whether the old observations 
were faulty or whether some unknown influence had 
acted on the planet. He had hardly made this invidi- 
ous distinction against the accuracy of the ancient 
observers when his own tables began to be out and 
grew seriously more so, so that within eleven years 
they quite failed to represent the planet. 

The discrepancies between theory and observation 
attracted the attention of the astronomic world, and the 
idea of another planet began to be in the air. The great 
Bessel was the first to state definitely his conviction in 
a popular lecture at Konigsberg in 1840, and thereupon 
encouraged his talented assistant Flemming to be- 
gin reductions looking to its locating. Unfortunately, 



THE OUTER PLANETS 121 

in the midst of his labors Flemming died, and shortly 
after Bessel himself, who had taken up the matter after 
Flemming's death. 

Somewhat later Arago, then head of the Paris observ- 
atory, who had also been impressed with the existence 
of such a planet, requested one of his assistants, a re- 
markable young mathematician named Leverrier, to un- 
dertake its investigation. Leverrier, who had already 
evidenced his marked ability in celestial mechanics, pro- 
ceeded to grapple with the problem in the most thor- 
ough manner. He began by looking into the pertur- 
bations of Uranus by Jupiter and Saturn. He started 
with Bouvard's work, with the result of finding it very 
much the reverse of good. The farther he went, the 
more errors he found, until he was obliged to cast it 
aside entirely and recompute these perturbations himself. 
The catalogue of Bouvard's errors he gave must have 
been an eye-opener generally, and it speaks for the 
ability and precision with which Leverrier conducted 
his investigation that neither Airy, Bessel, nor Adams 
had detected these errors, with the exception of one term 
noticed by Bessel and subsequently by Adams.* The 
result of this recalculation of his was to show the more 
clearly that the irregularities in the motion of Uranus 
could not be explained except by the existence of an- 
other planet exterior to him. He next set himself to 

* Adams, "Explanation of the Motion of Uranus," 1846. 



122 THE EVOLUTION OF WORLDS 

locate this body. Influenced by Bode's law, he began 
by assuming it to lie at twice Uranus' distance from 
the Sun, and, expressing the observed discrepancies in 
longitude in equations, comprising the perturbations 
and possible errors in the elements of Uranus, proceeded 
to solve them. He could get no rational solution. He 
then gave the distance and the extreme observations 
a certain elasticity, and by this means was able to 
find a position for the disturber which sufficiently satis- 
fied the conditions of the problem. Leverrier's first 
memoir on the subject was presented to the French 
Academy on November lo, 1845, ^^^^ gi^^'^g ^^^ place 
of the disturbing planet on June i, 1846. There is no 
evidence that the slightest search in consequence was 
made by anybody, with the possible exception of the 
Naval Observatory at Washington. On August 31 
he presented his third paper, giving an orbit, mass, 
and more precise place for the unknown. Still no 
search followed. Taking advantage of the acknowl- 
edging of a memoir, Leverrier, in September, wrote to 
Dr. Galle in Berlin asking him to look for the planet. 
The letter reached Galle on the 23d, and that very night 
he found a planet showing a disk just as Leverrier had 
foretold, and within 55' of its predicted place. 

The planet had scarcely been found when, on October 
I, a letter from Sir John Herschel appeared in the Lon- 
don AthencBum announcing that a young Cambridge 



THE OUTER PLANETS 123 

graduate, Mr. J. C. Adams, had been engaged on the 
same investigation as Leverrier, and with similar results. 
This was the first public announcement of Mr. Adams' 
labors. It then appeared that he had started as early 
as 1843, ^^^ ^^^ communicated his results to Airy in 
October, 1 845, a year before. Into the sad set of circum- 
stances which prevented the brilliant young mathema- 
tician from reaping the fruit of what might have been 
his discovery, we need not go. It reflected no credit on 
any one concerned except Adams, who throughout his 
life maintained a dignified silence. Suffice it to say 
that Adams had found a place for the unknown within 
a few degrees of Leverrier's; that he had communi- 
cated these results to Airy; that Airy had not consid- 
ered them significant until Leverrier had published an 
almost identical place; that then Challis, the head of 
the Cambridge Observatory, had set to work to search 
for the planet but so routinely that he had actually 
mapped it several times without finding that he had 
done so, when word arrived of its discovery by Galle. 
But now came an even more interesting chapter in 
this whole strange story. Mr. Walker at Washington 
and Dr. Petersen of Altona independently came to 
the conclusion from a provisional circular orbit for the 
newcomer that Lalande had catalogued in the vicinity 
of its path. They therefore set to work to find out 
if any Lalande stars were missing. Dr. Petersen 



124 THE EVOLUTION OF WORLDS 

compared a chart directly with the heavens to the 
finding a star absent, which his calculations showed 
was about where Neptune should have been at the 
time. Walker found that Lalande could only have 
swept in the neighborhood of Neptune on the 8th and 
loth of May, 1795. By assuming different eccentric- 
ities for Neptune's orbit under two hypotheses for 
the place of its perihelion, he found a star catalogued 
on the latter date which sufficiently satisfied his com- 
putations. He predicted that on searching the sky this 
star would be found missing. On the next fine even- 
ing Professor Hubbard looked for it, and the star was 
gone. It had been Neptune.* 

This discovery enabled elliptic elements to be com- 
puted for it, when the surprising fact appeared that 
it was not moving in anything approaching the orbit 
either Leverrier or Adams had assigned. Instead of a 
mean distance of 36 astronomical units or more, the 
stranger was only at 30. The result so disconcerted 
Leverrier that he declared that "the small eccentricity 
which appeared to result from Mr. Walker's compu- 
tations would be incompatible with the nature of the 
perturbations of the planet Herschel," as he called 
Uranus. In other words, he expressly denied that 
Neptune was his planet. For the newcomer pro- 
ceeded to follow the path Walker had computed. This 

* Proc. Amer. Acad., Vol. I, p. 64. 



THE OUTER PLANETS 125 

was strikingly confirmed by Mauvais' discovering that 
Lalande had observed the star on the 8th of May as 
v^ell as on the loth, but because the two places did 
not agree, he had rejected the first observation, and 
marked the second as doubtful, thus carefully avoiding 
a discovery that actually knocked at his door. 

Meanwhile Peirce had made a remarkable contri- 
bution to the whole subject. In a series of profound 
papers presented to the American Academy, he went 
into the matter more generally than either of the 
discoverers, to the startling conclusion **that the planet 
Neptune is not the planet to which geometrical analysis 
had directed the telescope, and that its discovery by 
Galle must be regarded as a happy accident." * He 
proved this first by showing that Leverrier's two 
fundamental propositions, — 

1. That the disturber's mean distance must be 
between 35 and 37.9 astronomical units; 

2. That its mean longitude for January i, 1800, must 
have been between 243° and 252°, — 

were incompatible with Neptune. Either alone might 
be reconciled with the observations, but not both. 

In justification of his assertion that the discovery 
was a happy accident, he showed that three solutions 
of the problem Leverrier had set himself were possible, 
all equally complete and decidedly different from 

* Proc. Amer. Acad., Vol. I, p. 65 et seq. 



126 THE EVOLUTION OF WORLDS 

each other, the positions of the supposed planet being 
120° apart. Had Leverrier and Adams fallen upon 
either of the other two, Neptune would not have been 
discovered.* 

He next showed that at 35.3 astronomical units, an 
important change takes place in the character of the 
perturbations because of the commensurability of period 
of a planet revolving there with that of Uranus. In con- 
sequence of which, a planet inside of this limit might 
equally account for the observed perturbations with the 
one outside of it supposed by Leverrier. This Nep- 
tune actually did. From not considering wide enough 
limits, Leverrier had found one solution, Neptune ful- 
filled the other.f And Bode's law was responsible for 
this. Had Bode's law not been taken originally as 
basis for the disturber's distance, those two great 
geometers, Leverrier and Adams, might have looked 
inside. 

This more general solution, as Peirce was careful 
to state, does not detract from the honor due either 
to Leverrier or to Adams. Their masterly calculations, 
the difficulty of which no one who has not had some 
experience of the subject can appreciate, remain as an 
imperishable monument to both, as does also Peirce's 
to him. 

* Proc. Amer. Acad., Vol. I, p. 144. 
f Proc. Amer. Acad., Vol. I, p. 332. 



CHAPTER V 

FORMATION OF PLANETS 

IN our first two chapters we saw what sign-posts 
in the sky there are pointing to the course evo- 
lution of a solar system probably follows, and sec- 
ondly, what evidence there is that our system took 
this road. We now come to a question not so easy 
to precise, — the actual details of the journey. It is 
always difficult to descend from a glittering panoramic 
survey to particular path-finding. The obstacles loom 
so much larger on a near approach. 

Most men shy at decisions and shun self-committal 
to any positive course, but when it comes to constructing 
a cosmogony, few at all qualified hesitate to frame one 
if the old does not suit. The safety in so doing lies in 
the fact that nothing in particular happens if it re- 
fuses to work. Its absurdity is promptly shown up, it 
is true, by some one else. For there is almost as good 
a trade in exposing cosmogonies as in constructing 
them. But no special opprobrium attaches to failure, 
because everybody has failed, from Laplace down, or 
up, as you are pleased to consider it. Besides it is 
really not so easy to do, as one is tempted to believe 

127 



128 THE EVOLUTION OF WORLDS 

before his book is published. Then only does the 
difficulty dawn, with a speed and clarity inversely 
proportional to the previous relation of the critic to the 
author. For the author himself is apt to be blind. 
With the fatal fondness of a parent for his offspring it 
is rare for the defects to be so glaringly apparent to 
their perpetrator. At the worst he considers them 
venial faults which can be glossed away. 

Attacking the subject in this judicial spirit, the reader 
can hardly expect me to satisfy him with a cosmogony 
entirely home-made, but at best to pursue a happymiddle 
course between creator and critic, advocating only such 
portions as happen to be my own, while sternly exposing 
the mistakes of others. 

In undertaking the hazardous climb toward the origin 
of things two qualities are necessary in the explorer : 
a quick eye for possibilities and a steady head in testing 
them. Without the discernment to perceive relations 
no ascent to first principles is possible; and without 
the support of quantitative criterion, one is in danger 
of becoming giddy from one's own imagination. Con- 
gruities must first hint at a path; physical laws then 
determine its feasibility. 

An eye for congruities is the first essential. For 
congruity alone accuses an underlying law. It is the 
analogic that with logic leads to great generalizations. 
Certain concords of the sort in the motions of the planets 



FORMATION OF PLANETS 129 

were what suggested to Laplace his system of the 
world. With the uncommon sense of a mathematician 
he perceived that such accordances were not necessitated 
by the law of gravitation, and on the other hand, could 
not be due to chance. The laws of probability showed 
millions to one against it. One of these happy harmon- 
ies was that all the large planets revolved about the Sun 
in substantially the same plane; another that they all 
travelled in the same sense (direction). Had they been 
unrelated bodies at the start, such agreement in motion 
was mathematically impossible. Their present con- 
sensus implied a common origin for all. In other 
words, the solar system must have grown to be what it 
is, not started so. 

This basic fact we may consider certain. But from 
it we would fain go on to find out how it evolved. To 
do so the same process must be followed. Considering, 
then, our solar system from this point of view, one can- 
not but be struck by some further congruities it presents. 
These are not quite those that inspired Laplace, because 
of discoveries since, and demand in consequence a theory 
different from his. 

The out about constructing a theory is that fresh 
facts will come along and knock for admission after 
the door is shut. They prove irreconcilables because 
they were not consulted in advance. The conse- 
quence is that since Laplace's time new relations have 

K 



130 THE EVOLUTION OF WORLDS 

come to light, and some supposed concords have had to 
be given up; so that were he ahve to-day he would him- 
self have formulated some other scheme. Two, how- 
ever, are still as true: that the planets all revolve in the 
same plane and in the same sense, and that sense that 
of the Sun's rotation. But so general a congruity as 
this points only to an original common moment of 
momentum and is equally explicable however that 
motion was brought about. It seems quite compatible 
with an original shock. To say that it was caused by a 
disruption is simply to go one step farther back than La- 
place. If, then, such a catastrophe did occur as the 
meteorites aver, we may perhaps draw some interesting 
inferences about it from the present state of the system. 
In a very close approach such as we must suppose for 
the disruption, one within Roches' limit of 2.5 diame- 
ters, the stranger, supposing him of equal size, would 
sweep from one side of the former Sun to the other in 
about two hours, and the brunt of the disrupting pull 
occur within that time. That the former Sun was ro- 
tating slowly seems established by the time, twenty- 
eight days, it now takes to go round. In which case 
the orbits of the masses which were to form the planets 
would all lie in about the same plane, — the plane of 
the tramp's approach. If there were exceptions, they 
should be found in the innermost. For such should 
partake most largely of the Sun's own original rota- 



^^^3, 



FORMATION OF PLANETS 131 

tion and travel therefore most nearly in its plane. 
And as a fact Mercury, the Benjamin, does differ from 
the others by revolving in a plane inclined some 7° to 
their mean, agreeing in this with the Sun's own rota- 
tion, with whose plane it was probably originally coin- 
cident (digression from it now being due to secular 
retrogression of the planets' nodes). ^ 

From the relations which advance has left unchanged 
we pass to those phenomena which seemed to present 
congruities in Laplace's day, but which have since 
proved void owing to subsequent detection of excep- 
tions. Time prevents my making the catalogue com- 
plete, but the reader shall be shown enough to satisfy 
him of the problem's complexity and to whet his desire 
for further research — on the part, preferably, of others. 

First comes, then, the rotations of the planets upon 
their axes, which Laplace supposed to be all in the same 



145' 



^^^^^_ ._ _ _ ,, j'- 

sun jupiter saturn uranus neptune 

Chart showing increasing tilts of the major planets. 

direction, counter to the hands of a clock; for the heav- 
ens mark time oppositely from us. All those within 
and including Saturn, the only ones he knew, turn, in- 
deed, in the same sense that they travel round the Sun. 
But Uranus departs from that direction by a right angle, 
wallowing rather than spinning in his orbit; while Nep- 



132 THE EVOLUTION OF WORLDS 

tune goes still farther in idiosyncratic departure and 
actually turns in the opposite direction. Here, then, 
Laplace's congruity breaks down, but in its place a little 
attention will show that a new one has arisen. For 
Saturn's tilt is ,27° and Jupiter's 3°, so that with the 
major planets there is revealed a systematic righting 
of the planetary axes from inversion through perpen- 
dicularity to directness as one proceeds inward toward 
the Sun. 

Another congruity supposed to exist a century ago 
was the exemplary agreement of all the satellites to fol- 
low in their planetary circuits the pattern set them by 
their primaries round the Sun. But as man has pene- 
trated farther into space and photographic plates have 
come to be employed, satellites have been revealed 
which depart from this orderly arrangement. This is 
the case with the ninth, the outermost, satellite of 
Saturn and with the eighth, the outermost, of Jupiter. 
But, as before, the breaking down of one congruity 
seems but the establishing of another. It appears that 
only the most distant satellites are permitted such un- 
conformity of demeanor. For departure from the 
supposed orthodoxy occurs in both instances where the 
distance is most, and does not occur in the case of all 
the other satellites found since Laplace's day, eleven 
in number, nearer their planets. 

A third congruity formerly believed in has suffered 



FORMATION OF PLANETS 



"^33 



a like fate; to wit, that satellites always moved in or 
near the equatorial plane of their primary. All those 
first discovered did; the four large ones of Jupiter, the 
main ones of Saturn, and probably those of Uranus and 



SATELLITE 

TILTS 
(ORBITAL) 



SATELLITE 

ECCENTRICITIES 

(ORBITAL) 



/ 

4 
VIII 



r VII 
'vi 



RETROGRADE 



JUPITER / 



DIRECT. 



_F7 ^K'-IV 

— — )ii mil 



SATURN 



lAPETUS 



••••••^"^ — 

TITAN 



vni 



JUPITER 



fVII 
. VI 



W'f\\Mv 
^iiii" 



SATURN 



TITAN 



lAPETUS 



^ 



Neptune. Even the satellites of Mars conformed, 
lapetus alone seemed to make exception, and that by a 
glossable amount. But this orderliness, too, has been 
disposed of, only, like the others, to experience a res- 
urrection in a different form. 

On examining more precisely the inclinations of 
these orbits some years ago, an interesting relation 
between them and the distances of the satellites from 
their primaries forced itself on my notice. The tilt 



134 THE EVOLUTION OF WORLDS 

increased as the distance grew. The only exceptions 
were very tiny bodies occupying a sort of asteroidal 
relation to the rest. 

A diagram will make this clear. The kernel of it 
dates from the lectures then delivered before the Massa- 
chusetts Institute of Technology in 1901. The inter- 
esting thing now about it is that the congruity there 
pointed out has been conformed to by every satellite 
discovered since, — the sixth, seventh, and eighth of 
Jupiter and the ninth and tenth of Saturn. It is evi- 
dent that we already know enough of the geniture of 
our system to prophesy something about it and have 
the prophecy come true. 

Closely connected with the previous relation is a 
fourth concordance clearly of mechanical origin, the 
relation of the orbital eccentricities of the satellites to 
their distances from their respective planets. The satel- 
lites pursue more and more eccentric orbits according 
as they stand removed from planetary proximity. 

A fifth congruity is no less striking. All the sat- 
ellites of all the planets that we can observe well enough 
to judge of turn the same face always to their lords. 
That the Moon does so to the Earth is a fact of every- 
day knowledge, and the telescope hints that the same 
respectful regard is paid by Jupiter's and Saturn's 
retinues to them. What is still more remarkable. 
Mercury and Venus turn out to observe the like vassal 



FORMATION OF PLANETS 135 

etiquette with reference to the Sun. And it will be 
noticed that they stand to him the nearest of his court. 
Here, then, is a law of proximity which points conclu- 
sively to some well-established force. 

Last is a remarkable congruity which study disclosed 
to me likewise some years ago, and which has received 
corroboration in discoveries since. This congruity is the 
peculiar arrangement of the masses in the solar system. 

Consider first the way in which the several planets, 
as respects size, stand ordered in distance from the 
sun. Nearest to him is Mercury, the smallest of all 
the principal ones. Venus and the Earth follow, each 
larger than the last; then comes Mars, of distinctly less 
bulk, and so to the asteroids, of almost none. After 
this the mass rises again to its maximum in Jupiter, 
and then subsequently falls through Saturn to Uranus 
and Neptune. Here we mark a more or less regular 
gradation between mass and position, a curve in which 
there are two ups and downs, the outer swell being 
much the larger, though the inner, too, is sufficiently 
pronounced. 

Now turn to Saturn and his family, which is the 
most numerous of the secondary systems and that 
having the greatest span. Under Saturn's wing, as it 
were, is the ring, itself a congeries of tiny satellites. 
Then comes Mimas, the smallest of the principal ones; 
then Enceladus, a little larger; then Tethys, the biggest 



136 THE EVOLUTION OF WORLDS 



of the three. Next stands Dione, smaller than Tethys. 
Then the mass increases with Rhea, reaching its cul- 
mination in Titan, after which it declines once more. 
Strangely reproductive this of the curve we marked in 



UPPER CURVE, 

OF DIAMETERS 

LOWER, OF MASS 



EACH SET 
DRAWN TO SCALE 

BUT SCALE 
DIFFERS IN EACH 




CURVES, 
GF DIAMETERS 



JUPITER SOLAR SYSTEM 



DISTANCE FROM JUPITER 

SATURN'S SYSTEM 

TITAN 



TO IX 




DISTANCE FROM SATURN 

URANUS' SYSTEM 



TITAN lA 



OBERON 



ARIEL 

UMBRIEL 



_DISJANCE_FROM URANU_S 

Masses of planets and satellites. 



% 



the arrangement of the planets themselves, even to the 
little inner rise and fall. 

Striking as such analogous ordering is, it is not all. 
For, scanning the Jovian system, we find the main curve 
here again; Ganymede, the Jupiter or Titan of the 
system, standing in the same medial position as they. 
Lastly, taking up Uranus and his family of satellites, 
the same order is observable there. Titania, the 
largest, is posted in the centre. 

Thus the order in which the little and the big are 
placed with reference to their controlling orb is the same 



FORMATION OF PLANETS 137 

in the solar system and in that of every one of its satel- 
Hte famihes. Method here is unmistakable. Nor is 
it easy to explain unless the cause in all was like. That 
the rule in the placing of the planets should be faith- 
fully observed by them in the ordering of their own 
domestic retinues, is not the least strange feature of 
the arrangement. It argues a common principle for 
both. Not less significant is the secondary hump in 
their distribution, denoting recrudescence farther in of 
the primary procedure shown without. 

One point to be particularly noticed in these latter- 
day congruities is that they are not simply general con- 
cords like the older ones — the fact that the planets 
move in one plane or in the same sense in that plane — • 
but detailed placings, ordered according to the dis- 
tances of the planets from the Sun or of the satellites 
from the planets. They are thus not simply of the 
combinative but of the permutative order of probabili- 
ties, a much higher one ; in other words, the chance that 
they can be due to chance is multiplicately small. Thus 
just as these analogies are by so much more remarkable, 
so are they by so much more cogent. They tell us not 
only of an evolution, but they speak of the very man- 
ner of its work. They do not simply generalize, they 
specify the mode of action. The difficulty is to under- 
stand their language. It is a case of celestial hiero- 
glyphics to which we lack the key. 



138 THE EVOLUTION OF WORLDS 

In attempting now to discover how all this came 
about we notice first that the system could not have 
originated in the beautifully simple way suggested by 
Laplace, because of several impossibilities in the path. 
If rings were shed, as he supposed, from a symmetric 
contracting mass, they should have resulted in some- 
thing even more symmetric than we observe to-day. 
In the next place they could not, it would appear, even 
if formed, have collected into planets. 

Nor could there have been an original *' fire-mist" 
with which as a stock in trade Laplace thriftily endowed 
his nebula to start with — the necessity for which has 
been likened to our supposed descent from monkeys; 
but which in truth is as misty a conception of the facts 
in the one case as it is a monkeying with them in the 
other. Darwin's theory distinctly avers that we were not 
descended from monkeys; and Laplace's fire-mist under 
modern examination evaporates away. It is an in- 
teresting outcome of modern analysis that the very fact 
which suggested the annular genesis of planets to La- 
place, the rings of Saturn, should now probably be 
deemed a striking instance of the reverse. Far from 
its being an exemplar in the heavens of the pristine state 
of the solar system, we may now see in it a shining pat- 
tern of how the devolution of bodies comes about. For 
instead of typifying an unfortunate set of particles which 
untoward circumstance has prevented from coalescing 



FORMATION OF PLANETS 139 

into a single orb, it almost certainly represents the dis- 
traught state to which a once more compact congeries 
of them has been brought by planetary interference. 
For to just such fate must the stresses in it caused by 
Saturn have eventually led. Disruption inevitable to 
such a group the observation of comets demonstrates 
is daily taking place. When a comet passes round the 
Sun or near a planet, the partitive pulls of the body 
tend to dismember it, and the same is a fortiori true of 
matter circulating round a planet as relatively near as 
the meteoric particles that constitute Saturn's rings. 
Starting as a congeries, it was pulled out more and more 
into a ring until it became practically even throughout. 
And the very action that produced it tends to keep it as 
surprisingly regular as we note to-day. 

No, the planets probably were otherwise generated 
and may have looked in their earlier stages as the knots 
in the spiral nebulae do to-day. But this does not mean 
that we can detail the process.^ 

Taking now the congruities for guide, we proceed to 
see what they affirm or negative. Laplace, when he 
ventured on his exposition of the system of the world, 
did so "with the mistrust which everything which is not 
the direct outcome of observation or calculation must 
inspire." To all who know how even figures can lie 
this caution will seem well timed. The best we can 
do to keep our heads steady is to lay firm hold at each 



140 THE EVOLUTION OF WORLDS 

step on the great underlying principles of physics. One 
of these is the conservation of the moment of momen- 
tum. This expression embodies one of the grandest 
generalizations of cosmic mechanics. The very phrase 
is fittingly sonorous, v^ith something of that religious 
sublimity which the dear old lady said she found such 
a consolation in the biblical word Mesopotamia. In- 
deed the idea is grand for its very simplicity. Mo- 
mentum means the quantity of motion in a body. It 
is the speed into the number of particles or the mass. 
Moment of momentum denotes the rotatory power of 
it round an axis. Now the curious and interesting 
thing about this quantity is that it can neither be di- 
minished nor increased. It is an abstraction from 
which nothing can be abstracted — but results. It is the 
one unalterable thing in a universe of change. What 
it was in the beginning in a system, that it forever re- 
mains. Because of this unchangeableness we can use it 
very effectively for purposes of deduction. One of 
these is in connection with that other great principle of 
physics, the conservation of energy. By the mutual 
action of particles on one another, by contraction, by 
tidal pulls, and so on, some energy of motion is con- 
stantly being changed into heat and thus dissipated 
away. Energy of motion, therefore, is slowly being 
lost to the system, and the only stable state for the 
bodies composing it is when their energy of motion has 



FORMATION OF PLANETS 141 

decreased to the minimum consistent with the initial 
moment of momentum. This principle we shall find 
very fecund in its application. It means that our whole 
system is evolving in a way to lessen its energy of motion 
while keeping its quantity of motion unchanged. The 
universe always does a thing with the least possible 
expenditure of force and gets rid of its superfluous 
energy by parting with it to space. Philosophers may 
wrangle over its being the best possible of worlds, but 
it is incontrovertibly mechanically the laziest, which 
a pessimistic friend of mine says proves it the best. 
Now this generalization finds immediate use in ex- 
plaining certain features of the solar system. In look- 
ing over the congruities it will be seen that deviation 
from the principal plane of the system or departure from 
a circular orbit is always associated with smallness in 
size. The insignificant bodies are the erratic ones. 
Now it has been shown mathematically in several dif- 
ferent ways that when small particles collect into a larger 
mass, the collisions tend to make the resultant orbit of 
the combination both more circular and more conform- 
ant to the general plane than its constituents. But 
we may see this more forthrightly by means of the 
general principle enunciated above. For in fact both 
results are direct outcomes of the conservation of mo- 
ment of momentum. Given a certain moment of mo- 
mentum for the system, the total energy of the bodies 



142 THE EVOLUTION OF WORLDS 

is least when they all move in one plane. This is evi- 
dent at once because the components of motion at right 
angles to the principal plane add nothing to the mo- 
ment of momentum of the system. It is also least when 
the bodies all revolve in circles about the centre of 
gravity. The circle has some interesting properties 
which almost justify the regard paid to it by the ancients 
as the only perfect figure. It encloses the maximum 
area for a given periphery, so that according to the 
old legends, if one were given as much land as he could 
enclose with a certain bull's hide, he should, after cutting 
the hide into strips, arrange these along the circumfer- 
ence of a circle. Now this property of the circle is 
intimately connected with the fact that a body revolving 
in a circle has the greatest moment of momentum for 
the least expenditure of energy. For under the same 
central force all ellipses of the same longest diameters — ■ 
major axes these are technically called — are described 
in the same time, and with the same energy, and of all 
such, the circle encloses the greatest area, which area 
measures the moment of momentum.^ 

Given a certain moment of momentum, then the 
energy is least when the bodies all move in one plane 
and all travel in circles in that plane. As energy is con- 
stantly being dissipated while any alteration among 
the bodies is going on, to coplanarity and circularity of 
path all the bodies must tend, if by collision they be ag- 



FORMATION OF PLANETS 143 

gregated into larger masses. As in the present state of 
our system the small bodies travel out of the general 
plane in eccentric ellipses while the big ones travel in it 
in approximate circles, the facts indicate that the origin 
of the larger masses was due to development by ag- 
gregation out of smaller particles. 

The next principle is of a different character. Half 
a century ago celestial mechanics dealt with bodies 
chiefly as points. The Earth was treated as a weighted 
point, and so was the Sun. This was possible because 
a sphere acts upon outside bodies as if all its mass 
were collected at its centre, and the Sun and many 
of the planets are practically spheres. But when it 
came to nicer questions of their present behavior and 
especially of their past career, it grew necessary to take 
their shape into account in their mutual effects. One 
of the results was the discovery of the great role played 
in evolution by tidal action. Inasmuch as the planets 
are not perfectly rigid bodies, each is subject to tidal 
deformation by the other, the outside being pulled 
more than the centre on one side and less on the other. 
Bodily tides are thus raised in it analogous to the sur- 
face tides we see in the ocean, only vastly greater, and 
these in turn act as a brake on its rotation. 

Now the retrograde motions occurring in the outer- 
most parts of all the systems, principal and subsidiary, 
only and always there: the retrograde rotations of 



144 THE EVOLUTION OF WORLDS 

Neptune and Uranus, the retrograde revolutions of the 
ninth satelhte of Saturn and of the eighth of Jupiter, 
point to something fundamental. For when we con- 
sider that it is precisely in its outer portions that any 
forces shaping the development of the system have had 
less time to produce their effect, we perceive that ap- 
parent abnormality now is really survival of the original 
normal state, only to be found at present in what has not 
been sufficiently forced to change. It suggests that the 
pristine motion of the constituents of the scattered 
agglomerations which went to form the planets was 
retrograde, and that their present direct rotations and 
the direct revolutions of most of their satellites have 
been imposed by some force acting since. Let us in- 
quire if there be a force competent to this end, and 
what its mode of action. 

Let us see how tidal action would work. Tidal force 
would raise bulges, and these, not being carried round 
with the planet's rotation except to a certain distance, 
due to viscosity, must necessarily act as brakes upon 
the planet's spin. In consequence of the friction they 
would thus exert, energy of motion must be lost. So 
long, then, as tidal forces can come into play, the energy 
of the system is capable of decrease. According to the 
last principle we considered, the system cannot be in 
stable equilibrium until this superfluous energy is lost 
or until tidal forces become inoperative, which cannot be 



FORMATION OF PLANETS 145 

till all the bodies in the system turn the same face to 
their respective centres of attraction. 

To see this more clearly, take the case of a retro- 
grade spin of a planet as compared with a direct one. 
The energy of the planet's spin is the same in both cases, 
because energy depends on the square of a quantity; 
to wit, that of the velocity, and is therefore independent 
of sign. Not so the moment of momentum. For this 
depends on the first power of the speed, and if positive 
in the one case, must be negative in the other. The 
moment of momentum of the whole system, then, is less 
in the former case, since the moment of momentum of 
the retrograde rotation must be subtracted from, that 
of the direct rotation be added to, that of the rest of the 
system. For a given initial moment of momentum 
with which the system was endowed at the start, 
there is, then, superfluous energy in the first state which 
can be got rid of through reduction to the second. 
Nature, according to her principles of least exertion, 
•avails herself of the chance of dispensing with it, and 
a direct rotation results. Sir Robert Ball first sug- 
gested this argument. 

Tidal action accomplishes the end. In checking up 
a body rotating contrary to the general consensus of 
spin, its first effect is to start to turn the axis over. For 
the body is in dynamical unstable equilibrium with 
regard to the rest of the system. The righting would 



146 THE EVOLUTION OF WORLDS 

continue, practically to the exclusion of any diminution 
at first of the spin, until the body had turned over in its 
plane so that the spin became direct. As the force in- 
creases greatly with nearness to the Sun, the effect would 
be most marked on the nearer, and most so on the biggest, 
bodies. This would account for the otherwise strange 
gradation from retrograde to direct in the tilts of the 
axes of the outer planets, and also for the present tilts 
of all the inner ones. 

Related to the initial retrograde rotations of the 
planets, and in a sense survivals from an earlier state of 
things, are two of the latest discoveries of motions in the 
solar system, the retrograde orbital movements of the 
ninth satellite of Saturn and the eighth of Jupiter. 
Considered so anomalous as scarcely at first to be be- 
lieved, it has been stated that they directly contradict 
the theory of Laplace. This is true; in the same 
sense and no more in which they directly contradict the 
contradictor, one of the latest theories. For neither 
theory has anything to explain them as the result of law. 
That they cannot be the sport of indifferent chance 
seems evidenced by their occupying similar external 
positions in their respective systems. As the product 
of a law we must regard them, and to find that law we 
now turn. Suppose the planet originally to have been 
rotating backward, or in the direction of the hands of a 
clock. At this time the satellite, which may never have 



FORMATION OF PLANETS 147 

formed a part of its mass, was travelling backward 
too, according to what we have said. Then under the 
friction of the tides raised on the planet by the Sun, the 
planet proceeded to turn over. It continued to do so 
until it spun direct. During this process there was no 
passage through zero of its moment of momentum con- 
sidered with regard to itself, and therefore no difficulty 
on that score of supposing that it successively generated 
satellites at all degrees of inclination. That its children 
are of the nature of adopted waifs, Babinet's criterion 
(1861) would seem to imply. But it must be remem- 
bered that the Sun has been slowing up the planet's 
rotation now for aeons. As it turned over, its tidal 
bulges tended to carry over with it such satellites as 
it already had. This effect was much greater on the 
nearer ones, both because they were nearer and be- 
cause they were much larger than the outer. So that 
the nearer kept with the planet, the others lagged pro- 
portionately behind. This suggests itself to account for 
the facts, but the subject involves so much that is un- 
certain that I submit the hypothesis with the distrust 
which Laplace has so eminently bespoken. I advance 
in its favor only the three striking facts : that a steady 
progression in their tilts of rotation is observable from 
Neptune to Jupiter and a substantially accordant one 
from Mars to Mercury; secondly, that the satellites 
turn their faces to their primaries, as likewise do Mer- 



148 THE EVOLUTION OF WORLDS 

cury and Venus to the Sun; and, thirdly, that the orbits 
of the satelHtes of all the planets are themselves tilted 
in accordance with what it would require.^ 

After the axial spins have been made over to the same 
sense, the second consequence of tidal action in the case 
of two bodies revolving about their common centre of 
gravity is to slow down both spins until first the smaller 
and then the larger turn the same face to each other 
and remain thus constant ever after. Now such is 
precisely the pass to which we observe the satellites 
of the planets have come. All that we can be sure of 
now turn the same face always to their primary. The 
Moon was the first to betray her attitude, because the 
one we can best note. On scrutiny, however, Jupi- 
ter's satellites, so far as we can make out, do the like; 
and Saturn's, too. And a very proper attitude it is, 
this regard paid to compelling attraction. Thus one of 
the congruities we noticed stands accounted for. The 
satellites could hardly have been at first so observant; 
time has brought about this unfailing recognition of 
their lords. 

Of the peculiar massing of the bodies in the family 
of the Sun, and the still stranger copying of it in their 
own domestic circles, little can as yet be said in in- 
terpretation. That the planetary families and their 
ancestral group should agree is not the least strange part 
of the affair. It shows that none of them was fortuitous. 



FORMATION OF PLANETS 149 

but that at the formation of all some common principle 
presided, apportioning the aggregations to their proper 
place. But it is such fine print of the system's history 
as at present to preclude discernment. 

So much for the details we may deduce of the method 
of our birth. We perceive unmistakably that our solar 
system grew to be what it is, and that it developed by 
agglomeration of its previously shattered fragments 
into the planets we behold to-day, but exactly how the 
process progressed we are as yet unable to precise. We 
are, however, as what I have mentioned and tabled 
show, every day accumulating data which will enable 
an eventual determination probably to be reached. 

From the fact of agglomeration, the essence of the 
affair, we turn to the traces it has left upon its several 
offspring. 

Just as the continued existence to-day of meteorites 
in statu quo informs us of a previous body from which 
our nebula sprang; so a physical characteristic of our 
own earth at the present time shows it to have evolved 
from that nebula — even though we cannot make out 
all the steps. Of its having done so, we are far more 
sure than of how it did. 

That primitive man perceived that somewhere below 
him was a fiery region which was not an agreeable 
abode, is plain from his consigning to such Tophet 
those whose religious tenets did not square with his 



150 THE EVOLUTION OF WORLDS 

own. That his conception of it was not strictly scien- 
tific is evidenced by his not realizing that to bury his 
enemies was the way to make them take the first step of 
the journey thither. Indeed, the vindictive venting of 
his notions clearly indicates their source as volcanic, 
rather than bred of a general disapproval of a down- 
ward descent either in silicates or sin. 

It was not till man began to bore into the Earth for 
metallic or potable purposes that he brought to light 
the generic fact that it was everywhere hotter as one 
went down. And this not only in a very regular, but in 
a most speedy, manner. The temperature increased 
in a really surprising way i° F. for every sixty-five feet 
of descent. As the rise continued unabated to the 
limit of his borings, becoming very unpleasant at its 
end, it was clear that at a depth of thirty-five miles 
even so refractory a substance as platinum must melt, 
and practically all the Earth except a thin crust be 
molten or even gaseous. 

Now heat, like money, is easy to dissipate but hard 
to acquire, as primitive man was the first to realize. It 
does not come without cause. Being a mode of motion, 
other motion must have preceded it from which it 
sprang. So much the doctrine of the conservation of 
energy teaches us, a doctrine considered now to have 
been the great scientific heirloom of the nineteenth 
century to the twentieth, yet which in its day caused the 



FORMATION OF PLANETS 151 

death of its first discoverer, Mayer, of a broken heart 
from non-recognition; its second, Helmholtz, was 
refused pubhcation by the leading BerHn physical 
magazine of the time. So quick is man to delay his 
own advance. 

The only conceivable motion for thus heating the 
Earth as a whole was the falling together of its parts. 
The present heat of the Earth, then, accuses the con- 
course of particles in the past to its formation, or in other 
words proves that the Earth was evolved out of material 
originally more sparcely strewn. It does so not only in 
a generic but in a most particular manner, for the heat 
is distributed just where it would be by such a process. 
It is greater to-day within, increasingly, because when 
the globe began to cool, the surface necessarily cooled 
first and established a regular gradient of heat from 
core to cuticle. 

It is possible to test this qualitative inference quanti- 
tatively and see if the falling together of the meteorites 
was equal to the task. Knowing the mechanical equiva- 
lent of heat, what we do is to calculate the quantity of 
motion involved and then evaluate it in heat. As we 
are unaware of the exact law of density of the Earth, and 
are ignorant of how much was radiated away in the 
process, the problem is a little like estimating the for- 
tune of a man when we do not know the stocks in which 
he has invested, and ignore how much he has spent the 



152 THE EVOLUTION OF WORLDS 

while. We only know what he would have been worth 
had he followed our advice in the matter of investments 
and lived as frugally as we recommended. For here, 
too, we are obliged to make certain assumptions. 
Nevertheless the figure obtained in the case of the plan- 
ets' stores of heat is so enormous as to leave a most 
ample margin for dissipation. Had the Earth con- 
tracted from a fairly generous expansion to its present 
state under the probable law of density suggested by 
Laplace in another connection, the heat developed 
would have been enough to raise the whole globe to 
1 60,000° F. if of iron, 90,000° F. if of stone. As 10,000° 
F. would have sufficed for the Earth to have kept up its 
past, to say nothing of its present, state, we are justified 
of our deduction. 

Nor is the Earth the only body in the system which 
thus argues itself evolved by the falling together of its 
present constituents. In the larger planets Jupiter and 
Saturn we seem to see the heat, far as we are away. 
For the cherry hue they disclose between their brighter 
belts proves to come from greater absorption there of 
the green and blue rays of the spectrum, indicating a 
greater depth of atmosphere traversed. Thus these 
parts lie at a lower level, and their ruddy hue is just what 
they should show were they still glowing with a dull red 
heat. 

Heat is not only the end of the beginning, it is the be- 





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FORMATION OF PLANETS 153 

ginning of the end as well. It is both the result of the 
evolving of definite bodies out of the agglomeration of 
matter-strewn space, and the cause of the higher evolu- 
tion of those globes themselves. For the acquisition of 
heat is the necessary preface to all that follows. Heat 
i's a body's evolutionary capital whose wise expenditure 
through cooling down makes all further advance to 
higher products possible. A body too small to have 
acquired it must remain forever lifeless, as dead as the 
meteorites themselves that enter our air as mere inert 
bits of stone or iron. 

Curiously enough, heat both must have been and then 
must have been lost. Like the loss of fortune or of 
friends sometimes in the ennobling of character, it is 
through its passing away that its effects are realized. 
For in cooling down from a once heated condition, that 
train of events occurs which we most commonly particu- 
larize as evolution. So far in our survey the march of 
advance has been through masses of matter, a molar 
evolution; from this point on it passes into its minute 
constituents and becomes a molecular one. The one 
is the necessary prelude to the other. Up to this great 
turning-point in the history of each member of a solar 
system we have been busied with the acquisition of heat, 
though we may not have been aware of it the while. 
All the motions we have studied tended to that end. 
During these three chapters, I, II, V, we have been 



154 THE EVOLUTION OF WORLDS 

gradually rising in our point of view until we stand at 
the temperature pinnacle of the whole process. In the 
next three we are to descend upon the other side. The 
slope we have come up was of necessity barren; the one 
we are to go down brings us to verdure and the haunts 
of men. Coming from the causes above, we reach at 
each step effects more and more related to ourselves 
which those causes will help us to explain. 



CHAPTER VI 



A planet's history 



Self-sustained Stage 

UP to this point in our retrospective survey the long 
course of evolution has taken one line, that of 
dynamical separation of the system's parts with sub- 
sequent reunitement of them according to the laws of 
celestial mechanics. Of this action I have submitted 
the reader my brief: departing in it from common-law 
practice, in which the cause of action is short and the 
brief long. And I have, I trust, guarded against his 
appealing on exceptions. 

From this point on we have two kinds of develop- 
ment to follow: the one intrinsic, the chemical; the 
other incidental, the physical. Not that, in a way, the 
one is divorcible from the other. For the physical 
makes possible the chemical by furnishing it the con- 
ditions to act. But in another sense, and that which is 
most thrust upon our notice, the two are independent. 
Thus oceans and land, hills and valleys, clouds and blue 
sky, as we know them, — everything, pretty much, 
which we associate with a world, — are not universal, 

155 



156 THE EVOLUTION OF WORLDS 

inevitable, results of planetary evolution, but resultant, 
individual, characteristics of our particular abode. 
They are as much our own as the peculiar arithmetic of 
w^aiters is theirs, or as used to be the sobriety of the 
country doctor's horse — his and no other's. Our 
w^hole geologic career is essentially earthly. Not that 
its fundamental laws are not of universal application, 
but the kaleidoscopic patterns they produce depend on 
the little idiosyncrasies of the constituents and the mode 
in which these fall together. Our everyday experiences 
we should find quite changed, could we alight on Venus 
or on Mars. 

On the other hand, the chemical changes which 
follow a body's acquisition of heat, setting in the mo- 
ment that heat has reached its acme and starts to de- 
cline, are as universal as the universe itself. They are 
conditioned, it is true, by the body's size and by the 
position that body occupied in the primal nebula, but 
they depend directly upon the degree of heat the body 
had attained. The larger the planet, the higher the 
temperature it reached and the fuller its possibilities. 
Even the planets are born to their estate. Thus the 
little meteorites live their whole waking life during the 
few seconds they spend rushing through our air. For 
then only does change affect their otherwise eternally 
inert careers. That the time is too short for any im- 
portant experience is evident on their faces. 



A PLANET'S HISTORY 157 

Heat is most intimately associated with the very con- \ 
stitution of matter. It is, in fact, merely the motion 
of its ultimate particles, and plays an essential part in 
their chemical relations. Just as a certain discreet 
fervor and sufficient exposure for attraction to take, 
ma'ke for matrimony, so with the little molecules, a 
suitable degree of warmth and a propitious opportunity 
similarly conduce to conjunction; too fiery a tempera- 
ment resulting in a vagabondage preventative of settled 
partnership and too cold a one in permanent celibacy. 
You may think the simile a touch too anthropomor- 
phic, but it is a most sober statement of fact. Indeed, 
it is more than probable that in some dull sense they 
feel the impulse, though not the need of expressing it in 
verse. That metals can remember their past states 
seems to have been demonstrated by Bose, and is cer- 
tainly in keeping with general principles as we know 
them to-day. For memory is the partial retention of 
past changes, rendering those changes more facile of 
repetition. 

A high degree of heat, then, makes chemical union 
impossible, because the great speeds at which the mole- 
cules are rushing past each other prevents any of them 
being caught. Lack of speed is equally deterrent. 
Nor is it wholly or even principally, perhaps, a move- 
ment of the whole which is here concerned, but a parti- 
tive throbbing of the molecule itself. Certain it is that 



158 THE EVOLUTION OF WORLDS 

great cold is as prohibitive of chemic combination as 
great heat. Phosphorus, which evinces such avidity for 
oxygen at ordinary temperatures as to have got its name 
from the way it pubhshes the fact, at very low ones 
shows a coolness for its affinity amounting to absolute 
unconcern. Thus only within a certain range of 
temperature does chemical combination occur. To re- 
main above or below this is to stay forever immortally 
dead. To get hot enough in the first place, and then 
subsequently to cool, are therefore essential processes 
to a body which is to know evolutionary advance. 

To pen the history of the solar system and leave out 
of it all mention of its most transcendentally wonderful 
result, the chemical evolution attendant upon cooling, 
would be to play ** Hamlet " with Hamlet left out. For 
the thing which makes the second half of the great cos- 
mic drama so inconceivably grand is the building up 
of the infinitely little into something far finer than the 
infinitely great. The mechanical action that first tore 
a sun apart, and then whirled the fragments into the 
beautifully symmetric system we behold to-day, is of a 
grandeur which is at least conceivable; the molecular 
one that, beginning where the other left off, built up 
first the diamond and then humanity is one that passes 
our power to imagine. That out of the aggregation of 
meteorites should come man, a being able to look back 
over his own genesis, to be cognizant of it, as it were, 



A PLANET'S HISTORY 159 

from its first beginnings, is almost to prove him immanent 
in it from the start. Fortunate it is that his powers 
should seem more limited than his perceptions, and the 
more so as he goes farther, else he had been but the 
embodiment of conceit. 

We must sketch, therefore, the steps in this marvel- 
lous synthesis; hastily, for I have already spoken of it 
elsewhere in print and repetitions dull appreciation, — - 
in the appreciative, — though we have the best of prec- 
edents for believing that, even in science, to be dull 
and iterative insures success; the dulness passing for 
wisdom and the iteration tiring opposition out. 

In the Sun all substances are in their elemental 
state. Though its materials are the same as the Earth's, 
we should certainly not feel at home there, even if we 
waived the question of comfort, for we should recog- 
nize nothing we know. We talk glibly of elements as 
if we had personal acquaintance with them, man's 
innate snobbery cropping out. For to the chemist 
alone are they observable entities. No one but he has 
ever beheld calcium or silicon, or magnesium, or man- 
ganese, and most of us would certainly not know these 
everyday elements if we met them on the street. Of 
all the substances composing the Earth's crust, or the air 
above, or the water beneath, practically the only ele- 
ments with which we are personally familiar are iron, 
copper, and carbon, and these only in minute quantities 



i6o THE EVOLUTION OF WORLDS 

and in that order of acquisition; which accounts for 
the stone, iron, and bronze ages of man, ending we 
may add with the graphite or lead-penciLage of early 
education. 

Yet that elementary substances once existed here we 
have evidence. We find such in volcanic vents. That 
the Earth was once as hot on its surface as it now is 
underneath, we know from the condition of the plutonic 
rocks where sedimentary strata have not covered them 
up. Volcanoes and geysers are our only avenues now 
to that earlier state of things. From these pathways 
to the past, and only from them, do we find elementary 
substances produced to-day, — hydrogen, sulphur, chlo- 
rine, oxygen, and carbon.* We are thus made aware 
that once the Earth was simple, too, on the surface as 
well as deeper down. A side-light, this, to what we 
knew must have been the case. 

From its primordial state, the least complex com- 
pounds were evolved first. As the heat lessened, higher 
and higher combinations became possible. And this is 
why the more complex molecules are so unstable, the 
organic ones the most. Since they are not possible at 
all under much stir of their atomic constituents, it 
shows that the bond between them must be feeble — 
and, therefore, easily broken by other causes besides 
heat. To the instability of the organic molecule is due 

* Geikie, *' Geology," pages 85, 86, and 131-136. 



A PLANET'S HISTORY i6i 

its power; and to cooling, the possibility of its expres- 
sion. 

For the steps in the chemical process from Sun to 
habitable Earth we must look to the spectroscope; not 
in its older field, the blue end of the spectrum, but in 
that which is unfolding to our view in Dr. Slipher's 
ingenious hands, the extension of the observable part of 
it into the red. For at that end lie the bands due to 
planetary absorption. Here we have already secured 
surprising results as to the atmospheres of the various 
planets. We have not only found positive evidence of 
water-vapor in the atmosphere of Mars, but we have 
detected strange envelopes in the major planets which 
show a constitution different from that of the Sun on the 
one hand, and of the Earth on the other. That size and 
position are for much in these peculiarities, I have al- 
ready shown you ; but something, too, is to be laid at the 
door of age. The major planets are not so advanced in 
their planetary history as is our Earth; and Dr. Sli- 
pher's spectrograms of them disclose what is now going 
on in that prefatory, childish stage. 

These spectrograms are full of possibilities, and it is 
not too much to say that chemistry may yet be greatly 
indebted to the stars. Compounds, the strange un- 
known substances there revealed by their spectral lines, 
may be cryptic as yet to us. Some of the elements miss- 
ing in Mendeleeff's table may be there, too. Helium 

M 



1 62 THE EVOLUTION OF WORLDS 

was first found in the Sun; coronium still awaits de- 
tection elsewhere. So with these spectral lines of the 
outer planets. It looks as if chemistry had been a 
thought too previous in making free for others with 
what should have been their names, Zenon and Ura- 
nium. For we may yet have to speak of Dion and 
Varunium. 

From the chemical aspect of evolution we pass to its 
physical side ; from the indirectly to the directly visible 
results. Here again, to learn what happened after the 
sunlike stage, we must turn to the major planets. 
For the cooling which induced both physical and chemi- 
cal change has there progressed less far, inasmuch as a 
large globe takes longer to cool than a small one. To 
the largest planets, then, we should look for types of the 
early planetary stages to-day. 

Almost as soon as the telescope was directed to 
Jupiter, among the details it disclosed were the Jovian 
belts (in the year 1630), dark streaks ruling the planet's 
disk parallel to its equator. They are of the first ob- 
jects advertised as visible in small glasses to-day, vying 
with the craters in the Moon as purchasable wonders of 
the sky. As the belts were better and better seen, fea- 
tures came out in them which proved more and more 
interesting. Cassini, in 1692, noticed that the mark- 
ings travelled round Jupiter and those nearest his 
equator the quickest. Sir William Herschel thought 



A PLANET'S HISTORY 163 

them due to Jovian trade-winds, the planet's swift 
rotation making up for deficiency of sun ; why, does not 
appear. 

Modern study of the planet shows that the bright lon- 
gitudinal layers between the dark belts are unquestion- 
ably belts of cloud. Their behavior indicates this, and 
their intrinsic brightness bears it out. For they are of 
almost exactly that albedo. Whether they are the kind 
of cloud with which we are familiar, clouds of water- 
vapor, we are not yet sure. But whatever their con- 
stitution, their conduct is quite other than is exhibited 
by our own. 

In the first place, they are of singular permanence for 
clouds. The fleeting forms we know as such assume 
in the Jovian air a stability worthy of Jove himself. In 
their general outlines, they remain the same for years 
at a time. "Constant as cloud" would be the proper 
poetic simile there. But while remaining true to them- 
selves, they prove to be in slow, unequal shift with one 
another. Thus Jupiter's official day difl^ers according 
to the watch of the particular belt that times it. Spots 
in diff'erent latitudes drift round lazily in appearance, 
swiftly in fact, those near the equator as a rule the fast- 
est. Nor is there any hard and fast latitudinal law; 
it is a go-as-they-please race in which one belt passes its 
neighbor at a rate sometimes of four hundred miles an 
hour. The mean day is 9^ 55™ long. 



1 64 THE EVOLUTION OF WORLDS 



X 



X 



A side-light is cast upon the Jovian state of things by 
the " great red spot," which has been more or less visible 

for thirty years, and which 
takes live minutes longer 
than the equatorial band to 
; travel round. Its tint be- 
/ spoke interest in what might 
be its atmospheric horizon. 
Yet it betrayed no sign of 
Jupiter and its "great red being either depressed or 

SPOT " A DRAWING BY Dr. 1 1 • 1 1 1 

Lowell, April 12, f^ om-sm, exalted With regard to the 
^9°^" rest of the surface. ** In 

1891," as Miss Gierke puts it, " an opportunity was 
offered of determining its altitude relative to a small 
dark spot on the same par- 
allel, by which, after months 
of pursuit, it was finally 
overtaken. An occultation I 
appeared to be the only alter- 
native from a transit; yet 
neither occurred. The 

dark spot chose a third. It Jupiter and its " GREAT RED 

SPOT " A drawing by Dr. 

coasted round the obstacle lowell, April 12, 7^ 28^-42^ 
in its way, and got damaged ^^°^" 
beyond recognition in the process." It thus astutely 
refused to testify. 

Now, this exclusiveness on the part of the " great 



"^Si 



V 



A PLANET'S HISTORY 



165 



red spot '' really offers us an insight to its character. 
Clearly it was no void, but occupied space with more 
than ordinary persistency. As it was neither above 
nor below the dark spot and shattered that spot on 



* •• 






'^ 






i^ 



^ 



Sun spots — after Bond. 



approach, which its former surroundings had not done, 
its force must have been due to motion. This can 
be explained by its being formed of a vast uprush of 
heated vapor from the interior. In short, it was a sort 
of baby elephant of a volcano, or geyser, occurring as 
befits its youth in fluid, not solid, conditions, but fairly 
permanent, nevertheless — a bit of kindergarten Jovian 
geology. This estimate of it is concurred in by Dr. 



1 66 THE EVOLUTION OF WORLDS 






1-1 * 







Slipher's spectrogram of the dark and light belts re- 
spectively. For in the spectrum of the dark one we see 

the distinctive 
Jovian bands in- 
tensified as if the 
light had trav- 
ersed a greater 
depth of Jovian 
air. Its color, a 
cherry red, abets 
the conclusion 
— that in such 
places we look 
down into the 
fiery, chaotic 
turmoil so inces- 
santly going on. 
It is of inter- 
est to note that 










Photograph of a sun spot — after the late we have protO- 

M. Tanssen. r 1 • 

types or this sort 
of extraterrestrial cyclone in the Sun. His spots are 
probably local upsettings of atmospheric equilibrium, 
using the word atmospheric in the widest possible 
sense. Just as our storms are the mildest examples 
of the like expostulation at the impossibility of keep- 
ing up a too long continued decorum. Only that with 



A PLANET'S HISTORY 167 

us the Earth is not so much to blame as the Sun; 
while both Jupiter and the Sun are themselves respon- 
sible for their condition. 

Thus we have, in the very depth of their negation, 
warrant from the dark belts of Jupiter that the bright 
ones are cloud. But also that they are not clouds 
ordered as ours. The Jovian clouds pay no sort of 
regard to the Sun. In orbital matters Jupiter obeys 
the ruler of the system; but he suffers no interference 
from him in his domestic affairs. His cloud-belts be- 
have as if the Sun did not exist. Day and night cause 
no difference in them ; nor does the Jovian year. They 
come when they will; last for months, years, decades; 
and disappear in like manner. They are sui Jovis, 
caused by vertical currents from the heated core and 
strung out in longitudinal procession by Jupiter's spin. 
They are self-raised, not sun-raised, condensations of 
what is vaporized below. Jove is indeed the cloud- 
compeller his name implies. 

Yet Jupiter emits no light, unless the cherry red of "^ ■/ 
his darker belts be considered its last lingering glow. ' 

He is thus on the road from Sun to world, and his pres- 
ent appearance informs us that this incubation takes 
place under cloud. 

The like is true of Saturn, in fainter replica, even to 
the cherry hue. In one way Saturn visibly asserts his 
independence beyond that possible by Jupiter. For 



1 68 THE EVOLUTION OF WORLDS 

Jupiter's equator lies almost in the plane of his orbit, and 
on a hasty view the Sun might be credited with the or- 
dering of the belts, as was indeed long the case. But 
Saturn's inclination to his orbital plane is 27°; yet his 
belts fit his figure as neatly as his rings, and never get 
displaced, no matter how his body be turned. 

Uranus and Neptune are in the same self-centred 
attitude at present as the faint traces of belts on their 
disk, otherwise of the same albedo as cloud, lead us to 
conclude. Yet both their densities and their situation 
give us to believe them further advanced than the giant 
I planets, and still they lie wrapped in cloud. 

These planets, then, are quite unbeholden to the Sun 
for all their present internal economies. What goes on 
under that veil of clouds with which they discreetly hide 
their doings from the too curious astronomic eye — we 
can only conjecture. But we discern enough to know 
that it is no placid uneventfulness. That it will con- 
tinue, too, we are assured. For whether these clouds 
are largely water-vapor now, or not, to watery ones 
they must come as the last of all the wrappers they 
will eventually put off. 

The major planets are the only ones at the present 
moment in this self-centred and self-sustained stage. 
Their great size has kept them young. In the smaller 
terrestrial planets we could not expect to witness any 
such condition to-day. If they experienced an ebullient 



A PLANET'S HISTORY 



169 



youth, they have long since outgrown it. Only by 
rummaging their past could we find evidence on the 
point, and this, distance both in time and space bars 






«X. ^r^it-f^U^MitA* -14 t9*i.* ftMfi.fi 



The volcano Colima, Mexico, March 24, 1903 — Jose 
Maria Arreola, per Frederick Starr. 

us from doing. There is but one body into whose 
foretime career we could hope to peer with the slightest 
prospe'ct of success — our own Earth. 

Whether our Earth was ever hot enough at the surface 
to vaporize those substances which now form the Jovian 



170 THE EVOLUTION OF WORLDS 



or Saturnian clouds, we do not know; but that it was 
once hot enough to vaporize water we are perfectly 
certain. And this from proof both of what did exist 




Jukes Butte, a denuded laccolith, as seen from the 
northwest gilbert. 

and of what did not. That the surface temperature 
was at one time in the thousands of degrees Fahrenheit, 
the Plutonic magma underlying all the sedimentary 
rocks of the Earth amply shows. Reversely, the ab- 
sence of any effect of water until we reach these sedi- 
mentary deposits, testifies that during all the earlier 
stages of the Earth's career water as such was absent, 
and as water subsequently appeared, it is clear that the 

conditions did not at first 
allow it to form. We are 
sure, therefore, that there 
was a time when water ex- 
Ideal section of a LACCOLITH— istcd ouly as steam, and very 

Gilbert. . 

possibly a period still an- 
terior to that when it did not exist at all, its constitu- 
ent hydrogen and oxygen not having yet combined. 
There was certainly an era, then, in the morning of the 




A PLANET'S HISTORY 171 

ages, when the Earth wore her cloud-wrapper much as 
Jupiter his now. 

That the seas were not once and yet are to-day, affords 
proof positive that at some intermediate period they be- 
gan to be. Avery long intermediate one it must have 
been, too, — all the time it took the Earth to cool from 
about 2000° C. to 100° C. Not till after the temperature 
had fallen to the latter figure in the outer regions of the 
atmosphere could clouds form, and not till it had done 
so at the solid surface could the steam be deposited as 
water. Reasoning thus presents us with a picture of 
our Earth as a vast seething; caldron from which steam 
condensing into cloud was precipitated upon a heated 
layer of rock, to rise in clouds of steam again. The 
solid surface had by this time formed, thickening slowly 
and more or less irregularly, and into its larger dimples 
the water settled as it grew, deepening them into the 
great ocean basins of to-day. We see the process with 
as much certainty and considerably more comfort than 
if, in the French sense, we had assisted at it. Presence 
of mind now thus amply makes up for absence of body 
then. 

Passing on evolutionarily we reach more and more 
tolerable conditions and solid ground in fact, as well as 
theory. Thus the crust hardened and cooled, while the 
oceans still remained uncomfortably hot. For water 
requires much more heat to warm it to a given tempera- 



172 THE EVOLUTION OF WORLDS 

ture than rock, about four and a half times as much. 
It has therefore by so much the more to lose, and is pro- 
portionally long in the losing. These hot seas must 
have produced a small universe of cloud, and as the 
conditions v^ere the same all over the Earth, we can see 
easily with the mind's eye that we could not have seen 
at all with the bodily one, had we occupied the land in 
those very early days. To be quite shut out from curi- 
ous sight without, was hardly made up for by not being 
able to see more than dimly within. Any one who has 
stood on the edge of a not-extinct crater when the wind 
was blowing his way, will have as good a realization of 
the then state of things as he probably cares for. 

Now this astronomic drawing of the then Earth, 
which by its lack of detail allows of no doubt whatever, 
permits us to offer help in the elucidation of some of 
their phenomena to our geologic colleagues. We are 
the more emboldened to do so in that they have them- 
selves appealed to astronomy for diagnosis, and accepted 
nostrums devised by themselves. It is always better in 
such cases to call in a regular practitioner. Not that he 
is necessarily more astute, but that he knows what will 
not work. It was in the matter of the paleologic climate 
that they were led to consult astronomy. The singular 
thing about paleologic times was the combination of 
much warmth with little light; and the not less singular 
fact that these conditions were roughly uniform over 



A PLANET'S HISTORY 173 

the whole Earth. From this universahty it was clear, 
as De Lapparent, their chief spokesman, puts it, that 
nothing local could explain the fact. It was something 
which demanded a cause common to the globe. 

It thus fell properly within the province of astronomy. 
For if we are to draw any line between the spheres of 
influence of the two sciences, it would seem to lie where 
totality ends and provincialism begins. I use this not 
as a pejorative, but simply to part local color from one 
universal drab. In the Earth's general attributes, — its 
size, shape, and weight, — we must have recourse to 
astronomy to learn the facts. Not less so for those prin- 
cipal causes which have shaped its general career; we 
surrender it only at the point where everyday interest 
begins, when those causes that led it through its uninvit- 
ing youth give way to effects which in the least concern 
humanity at large. 

Between the mere aggregation of matter into planet- 
ary bodies, of which nebular hypotheses treat, and the 
specific transformation of plants and animals upon 
their surfaces with which organic evolution is concerned, 
lies a long history of development, which, beginning at 
the time the body starts to cool, continues till it be- 
come, for one cause or another, again an inert mass. 
In this period is contained its career as a world. 
Planetology I have ventured to call the brand of as- 
tronomy which deals with this evolution of worlds. It 



174 THE EVOLUTION OF WORLDS 

treats of what is general and cosmic in that evolution, 
as geology treats of what is terrestrial and specific in 
the history of one member of the class, our own Earth. 
The two do not interfere, as the one faces questions in 
time and space to which the other remains perforce a 
stranger. If the picture by the one be fuller of detail, 
the canvas of the other permits of the wider perspec- 
tive. Certain events in the history of our Earth can 
only be explained by astronomy, as geologists have 
long since recognized. It is these that fall into our 
present province. 

Geologists, however, have applied astronomy accord- 
ing to their own ideas. Either they called in aurists, so 
to speak, when what they needed was an oculist, or they 
went to books for their drugs, which they then ad- 
ministered themselves — a somewhat dangerous prac- 
tice. Thus they began by displacing the Earth's axis 
in hope of effecting a result; not realizing that this 
would only shift the trouble, not cure it; in fact, make 
it rather worse. They next tried what De Lapparent, 
one of the most brilliant geologists of the age, calls ** a 
variation in the eccentricity of the ecliptic * joined to 
precession of the equinoxes," — a startling condition un- 
known to astronomy which does not deal in eccentric 
planes, whatever such geometric anomalies may be, but 
by which its coiner evidently means a change in the 

* " Abrege de Geologic," De Lapparent. 



A PLANET'S HISTORY 175 

eccentricity of the orbit, as the context shows. Its 
effect on the Earth, as he wisely points out, would be to 
reduce its extremities to extremes. To get out of his 
quandary he then embraced a brilliant suggestion of a 
brother geologist, M. Blandet. M. Blandet conceived 
the idea, and brought it forth unaided, that all that was 
necessary was a sun big enough to look down on both 
poles of the Earth at once. To get this he travelled back 
to the time when, in Laplace's cosmogony, the Sun filled 
the whole orbit of Mercury. This conception, which, 
De Lapparent remarks, ** might, at the time of its ap- 
parition, have disconcerted spirits accustomed to con- 
sider our system as stable," — an apparition which we 
may add would certainly continue to disconcert them, — 
he says seems to him quite in harmony with that sys- 
tem's genesis. That it labors under two physical im- 
possibilities, one on the score of the Sun, the other on 
that of the Earth, and that in this case two negatives 
do not make an affirmative, need not be repeated here, 
as the reader will find it set forth at length elsewhere,"^ 
together with what I conceive to be the only explana- 
tion of paleothermal times which will work astronomi- 
cally — presently to be mentioned. But before I do 
so, it is pertinent to record two things that have come to 
my notice since. One is that in rereading Faye's 
" Origine du Monde," I came upon a passage in which 

* " Mars as the Abode of Life," Macmillan, 1908. 



176 THE EVOLUTION OF WORLDS 

it appears that M. Blandet had actually consulted Faye 
about his hypothesis, and that Faye had shown him its 
impossibility on much the same grounds as those above 
referred to; which, however, did not deter M. Blandet 
from giving it to the world nor De Lapparent from god- 
fathering the conception. 

Faye, meanwhile, developed his theory of the origin 
of the world, and by it explained the greater heat and 
lesser light of paleologic times compared with our own, 
thus : The Earth evolved before the Sun. In paleologic 
times the Sun was still of great extent, — an ungathered- 
up residue of nebula that had not yet fallen together 
enough to concentrate, not a contracting mass from 
which the planets had been detached, — and was in 
consequence but feebly luminous and of little heating 
effect; so that there were no seasons on Earth and no 
climatic zones. The Earth itself supplied the heat 
felt uniformly over its whole surface. 

This differs from my conception, as the reader will 
see presently, in one vital point — as to why the Earth 
was not heated by the Sun. In the first place Faye's 
sun has no raison d'etre ; and in the second no visible 
means of existence. If its matter were not already 
within the orbit of the Earth at the time, there seems no 
reason why it should ever get there ; and if there, why 
it should have been so loath to condense. We cannot 
admit, I think, any such juvenility in the Sun at the time 




Tree fern. 



A PLANET'S HISTORY 177 

the Earth was already so far advanced as geology shows 
it to have been in paleologic times. For the Earth had 
already cooled below the boiling-point of water. 

To understand the problem from the Earth's point of 
view, let us review the facts with which geology presents 
us. The flora of paleologic times, as we see both at 
their advent in the Devonian and from their superb 
development in the Carboniferous era, consisted wholly 
of forms whose descendants now seek the shade.* Tree 
ferns, sigillaria, equisetae, and other gloom-seeking 
plants composed it. That some tree-fern survivals to- 
day can bear the light does not invalidate the racial ten- 
dency. We have plenty of instances in nature of such 
adaptability to changed conditions. In fact, the dying 
out and deterioration of most of the order shows 
that the conditions have changed. And these plants, 
grown to the dimensions of trees, inhabited equally the 
tropic, the temperate, and the frigid zones as we know 
them now. Lastly, no annual rings of growth are to be 
found on them.j- In other words, they grew right on, 
day in, day out. The climate, then, was as continuous 
as it was widespread. 

On the other hand, astronomy and geology both as- 
sert that the seas were warm.f From this it follows 

* De Lapparent, Dana, Geikie, passim. 

f De Lapparent. 

X De Lapparent, Dana, Geikie, passim. 

N 



178 THE EVOLUTION OF WORLDS 

that a vastly greater evaporation must have gone on 
then than now, and that a welkin of cloud must thus 
inevitably have been formed. 

Now put the two facts together, and you have the solu- 
tion. The climate was warm and equable over the whole 
globe because a thick cloud envelope shut off the Sun's 
heat, the heat being wholly supplied from the steamy 
seas. At the same time, by the same means the light 
was necessarily so tempered as to produce exactly that 
half-light the ferns so dearly love. One and the same 
cause thus answers the double riddle of greater warmth 
and less light in those old days than is now the case. 

And here comes in the second find I spoke of above, 
in the person of some old trilobites who stepped in un- 
expectedly in corroboration. It has long been known 
— though its full significance seems to have escaped 
notice — that in 1872 M. Barrande made the discovery 
that many species of trilobites of the Cambrian and 
lower Silurian, the two lowest, and therefore the oldest, 
strata of paleozoic times, and distant relative of our 
horseshoe crabs, were blind. What is yet more signifi- 
cant, the most antediluvian were the least provided with 
eyes. Thus in the primordial strata, one-fourth of 
the whole number of species were eyeless, in the next 
above one-fifth, and in the latest of all one two-hundredth 
only.* Furthermore, they testify to the difficulty of 

* Suess, " The Face of the Earth," p. 213. 



A PLANET'S HISTORY 179 

seeing, in two distinct ways, some by having no eyes 
and some colossal ones, strenuous individuals increasing 
their equipment and the lazy letting it lapse. It seems 
more than questionable to attribute this blindness to a 
deep-sea habitat, as Suess does in describing them, 
for they lived in what geologists agree were shallow 
seas on the site of Bohemia to-day. Besides, trilobites 
never had abyssal proclivities; for they are found pre- 
served in littoral deposits, not in deep-sea silt. Muddy 
water may have had some hand in this, but muddy 
water itself testifies to great commotion above and tor- 
rential rains. So the light in those seas was not what it 
became later, or would be now. Thus these trilobites 
were antelucan members of their brotherhood, and this 
accuses a lack of light in those earlier eras even greater 
than in Carboniferous times, which is just where it 
ought to be found if the theory is true. 

I trust this conception may prove acceptable to geol- 
ogists, for it seems imperative from the astronomic 
side that something of the sort must have occurred. 
And it is just as well, if not better, to view it thus in the 
light of the dawn of geologic history as to remain in the 
dark about it altogether. Nescience is not science — 
whether hyphenized or apart; for the whole object of 
science is to synthesize and explain. Its body of learn- 
ing is but the letter, coordination the spirit, of its law. 
Nevertheless, the unpardonable impropriety of a new 



i8o THE EVOLUTION OF WORLDS 

idea, I am awaiC, is as reprehensible as the atrocious 
crime of being a young man. Yet the world could not 
get on without both. Time is a sure reformer and 
will render the most hardened case of youth senile in 
the end. So even a new idea may grow respectable at 
last. And it is really as well to make its acquaint- 
ance while it still has vigor in it as to wait till it is 
old and may be embraced with impunity. Boasted 
conservatism is troglodytic, and usually proves a 
self-conferred euphuism for dull. For conservatism 
proceeds from slowness of apprehension. It may be 
necessary for certain minds to be in the rear of the 
procession, but it is of doubtful glory to find distinction 
in the fact. 

Thus the youth of a world, like the babyhood of an 
individual, is passed screened from immediate contact 
from without. That this is the only way that life can 
originate on a planet we cannot say, but that it is away 
in which it does occur, our own Earth attests, and 
that, moreover, it is the way with all planets of sufficient 
size, the present aspect of the major planets shows. It 
may well be that with celestial bodies as with earthly 
species, some swaddle their young, others cast them 
forth to take their chance, and that those that most pro- 
tect them rear the higher progeny in the end. What 
glories in evolution thus await the giant planets when 
they shall have sufficiently cooled down, we can only 



A PLANET'S HISTORY i8i 

dimly imagine. But we can foresee enough to realize 
that we are not the sum of our solar system's possibili- 
ties, and by studying the skies read there a future more 
wonderful than anything we know. 



CHAPTER VII 



A planet's history 



Sun-sustained Stage 

TWO stages have characterized the surface history 
of the Earth, — stages which may be hkened to the 
career of the chick within and without the egg. In the 
first of them the Earth lay screened from outside in- 
fluence under a thick shell of cloud, indifferently ex- 
clusive of the cold of space or of the heating beams of the 
Sun. Motherless, the warmth of its own body brooded 
over it, keeping its heat from dissipating too speedily 
into space, and so fostering the life that was quickening 
upon its surface. 

The second stage began when the egg-shell broke and 
the chick lay exposed to the universe about it, to get its 
living no longer from its little world within, but from 
the greater one without. One and the same event ended 
the old life to make possible the new. So soon as the 
cloud envelope was pierced, both the Earth's own heat 
escaped and the Sun's rays were permitted to come in. 

It is not surprising that under such changed condi- 
tions development itself should have changed, too. In 

182 



A PLANET'S HISTORY 183 

fact, the transformation was marked. That its epochal 
character has failed to impress itself generally on 
geologists, is perhaps because they look too closely, 
missing the march of events in the events themselves. 




Earth as seen from above — Photographed by Dr. Lowell at an 

altitude of 550o feet. 

and because, too, of the gradual nature of its proces- 
sional change. We can recall only De Lapparent as 
having particularly signalled it; although not only in 
its cause, but for its effects, it should have delimited 
two great geologic divisions of time. 

Astronomy and geology are each but part of one uni- 
versal history. The tale each has to tell must prove in 
keeping with that of the other. If they seem at vari- 
ance, it behooves us very carefully to scan their respec- 
tive stories to find the flaw where the apparent incon- 
gruity slipped in. Each, too, fittingly supplements the 
other, and especially must geology look to astronomy 



1 84 THE EVOLUTION OF WORLDS 

for its initial data, since astronomy deals with the be- 
ginning of our own Earth. 

That study of our Earth in its entirety falls properly 
within the province of astronomy, is not only deducible 
from its relationship to the other planets, but demon- 
strable from the cosmic causes that have been at work 
upon it, and the inadequacy of anything but cosmic laws 
to explain them. The ablest geologists to-day are be- 
coming aware of this, — we have one of them at the 
head of the geology department of the Institute, — 
while from the curious astronomy at second hand which 
gets printed in geologic text-books, by eminent men 
at that, dating from some time before the flood, — of 
modern ideas, — it seems high time that the connection 
should be made clear. 

For, after all, our Earth too is a heavenly body, in 
spite of man's doing his best to make it the reverse. It 
has some right to astronomic regard, even if it is our own 
mother. At the same time it is quite puerile to consider 
the universe as bounded by our terrestrial backyard. 
If man took himself a thought less importantly, he might 
perceive the humor of so circumscribed a view. Like 
children we play at being alone in the universe, and then 
go them one better by believing it too. 

I shall, of course, not touch on any matters purely 
geologic, for fear of committing the very excesses I 
deplore; mentioning only such pKDints as astronomy has 



A PLANET'S HISTORY 185 

information on, and which, by the sideHghts it throws, 
may help to illuminate the subject. 

Thus it certainly is interesting and may to many be a 
new point of view, that the changes introduced when 
paleologic times passed into neologic ones were in their 
fundamental aspects essentially astronomic; which 
shows how truly astronomic causes are woven into the 
whole fabric of the Earth. For it was then only, 
terrestrially speaking, that the year began. The or- 
bital period had existed, of course, from the time the 
Earth first made the circuit of the Sun. But the year 
was more a succes d^ estime on the Sun's part than one of 
popular appreciation. As the Sun could not be seen 
and worked no striking effects upon the Earth, the 
annual round had no recognizable parts, and one revo- 
lution lapsed into the next without demarcation. Only 
with the clearing of the sky did the seasons come in: 
to register time by stamping its record on the trees. 
Before that, summer and winter, spring and autumn, 
were unknown. 

Climate, too, made then its first appearance; climate, 
named after the sunward obliquity of the Earth, and 
seeming at times to live down to that characterization. 
Weather there had been before; pejoratively speaking, 
nothing but weather. For the downpours in paleo- 
logic times must have been exceeded in numbers only 
by their force. One dull perpetual round of rain was 



1 86 THE EVOLUTION OF WORLDS 

the programme for the day, with absolutely no hope of 
a happy clearance to-morrow. It was the golden age 
only for weather prophets whose prognostications could 
hardly go wrong. With climate, however, it was a very 
different matter. With polyp corals building reefs 
almost to the pole (8i° 50'),* as far north nearly as man 
has yet by his utmost efforts succeeded in getting, while 
their fellows were busy at the like industry in the 
tropics, it is clear that latitude was laughed at and cli- 
mate even lacked a name. 

Another astronomic feature, then for the first time 
disclosed, was the full significance of the day and the 
revelation of its cause. While the Earth brooded under 
perpetual cloud, there could have been but imperfect 
recognition of day and night. Or perhaps we may put 
it better by saying that the standard of both was greatly 
depressed, dull days alternating with nights black as 
pitch. But the moment the Sun was let in, all this 
changed, though not in a twinkling. The change came 
on most gradually. We can see in our mind's eye the 
first openings in the great welkin permitting the Earth 
its initial peeps of the world beyond, and how quickly 
and tantalously they shut in again like a mid-storm 
morning which dreams of clearing only to find how 
drowsy it still is. But eventually the clouds parted 
afresh and farther, and the Earth began to open its 
eyes to the universe without. 

* Dana, " Geology." 



A PLANET'S HISTORY 187 

The cause of the clearing, of course, was the falHng 
temperature of the seas. Evaporation went on much 
less fast as the heat of the water lessened. The whole 
round of aquatic travel from ocean to air, and back to 
ocean again, proceeded at an ever slackening pace. And 
here, if it so please geologists, may be found a reconcil- 
ing of their demands for time to the relative pittance 
astronomy has been willing to dole them out, a paltry 
50 or 100 millions of years, which like all framers of 
budgets they have declared utterly insufficient. For 
in early times the forces at work were greater, and 
by magnifying the means you quicken the process and 
contract the Earth's earlier eras to reasonable limits. 

Upon these various astronomic novelties, the Earth 
on thus awakening looked for the first time. Such re- 
gard altered for good its own internal relations. The 
wider outlook made impossible the life of the narrower 
that preceded it. A totally changed set of animals and 
plants arose, to whom the cosmos bore a different as- 
pect. The Earth ceased to be the self-centred spot it 
seemed before. As long ago as this had the idea that 
our globe was the centre of the universe been cosmi- 
cally exploded. The Earth knew it if man did not. 

Its denizens responded. The organisms that already 
inhabited it proceeded to change their character and 
crawl out upon the land. For in Devonian times the 
Earth was the home of fishes. The land was not con- 



1 88 THE EVOLUTION OF WORLDS 

sidered a fit abode by anything but insects, and not over- 
good by them. But it looked different when the Sun 
shone. Some maritime dwellers felt tempted to ex- 
plore, and proceeded in the shape of amphibians to spy 




Tracks of Sauropus prim^vus (x 1). I. Lea. — Dana, "Manual op 

Geology." 

out the land. They have left very readable accounts 
of their travels in foot-notes by the way. As one should 
always inspect the original documents, I will reproduce 
the foot-notes of one early explorer. It is one of the few 
copies we have, as the type is worn out. But it tells a 
pretty full story as it stands. The ripple-marks show 
that a sea beach it was which the discoverer trod in his 
bold journey of a few feet from home and friends, and the 
pits in the sandstone that it was raining at the time of his 
excursion. No Columbus or Hakluyt could have left 
a record more precise or more eminently trustworthy. 
The pilgrims found it so good that their eventual 
collaterals, the great reptiles, actually took possession 
of the land and held it for many centuries by right of 
eminent domain. Yet throughout the time of these 



A PLANET'S HISTORY 189 

bold adventurers, their skies were only clearing, as the 
pitting of the sandstone eloquently states. 

It was not till the chalk cliffs of Dover were being 
laid down that we have evidence that seasons had fully 
developed, in the shape of the first deciduous trees.* 
Cryptogams, cycads, and, finally, conifers had in turn 
represented the highest attainments of vegetation, and 
the last of these had already recognized the seasons by 
a sort of half-hearted hibernation or annual moulcing; 
deeming it wise not to be off with the old leaves before 
they were on with the new. But finally the most ad- 
vanced among them decided unreservedly to accept the 
winter and go to sleep till spring. The larches and 
ginkgo trees are descendants of the leaders of this 
coniferous progressive party. 

At the same time color came in. We are not ac- 
customed to realize that nature drew the Earth in grays 
and greens, and touched it up with color afterward. 
Only the tempered tints of the rocks and the leaden 
blue of the sea, subdued by the disheartening welkin 
overhead to a dull drab, enlivened their abode for the 
oldest inhabitants. But with Tertiary times entered 
the brilliantly petalled flowers. Beginning with yellow, 
these rose through a chromatic scale of beauty from 
white through red to blue.f They decked themselves 

* Dana, Geikie, De Lapparent. 
t Cf. Grant Allen. 



190 THE EVOLUTION OF WORLDS 

thus gaudily because the Sun was there to see by, as 
well as eyes to see. For without the Sun those uncon- 
scious horticulturists, the insects, could not have exer- 
cised their pictorial profession. 

To the entering of the Sun upon the scene this won- 
drous revolution was due; and once entered, it became 
the dominant factor in the Earth's organic life. We 
are in the habit of apostrophizing the Sun as the source 
of all terrestrial existence. It is true enough to-day, and 
has been so since man entered on the scene. But it was 
not always thus. There was a time when the Sun 
played no part in the world's affairs. 

As its heat is now all-important, it becomes an in- 
teresting matter to determine the laws governing its 
amount. That summer is hotter than winter we all 
know from experience, pleasurable or painful as the 
case may be. This is due to the fact that the Sun is 
above the horizon for a greater number of hours in 
summer and passes more directly overhead. But not so 
many people are aware that on midsummer day, so far 
as the Sun is concerned, the north pole should be the 
hottest place on earth. That Arctic explorers, who 
have got within speaking acquaintance of it, assure us 
it is not so, shows that something besides the direct rays 
of the Sun is involved. Indeed, we learn as much from 
the extensively advertised thermometers of winter re- 
sorts which, judiciously placed, beguile the stranger to 



A PLANET'S HISTORY 191 

sojourn where it is just too cold for comfort. The 
factor in question is the blanketing character of our air. 
Now a blanket may keep heat out as well as keep it in. 
Our air acts in both capacities. It is by no means 
simply a storer of heat, as many people seem to suppose ; 
it is a heat-stopper as well. What it really is is a tempo- 
rizer, a buffer to ease the shocks of sudden change like 
those comfortable, phlegmatic souls who reduce all emo- 
tion to a level. For the heating power of the Sun, 
even at the Earth's distance away, is much greater than 
appears. Knowledge of this we owe most to Langley, 
and then to Very, who continued his results to yet a 
finer determination, the best we have to-day. In con- 
sequence we have learnt that the amount of heat we 
should receive from the Sun, could we get above our air, 
— the solar constant, as it is called, — would be over 
three times what it is on the average in our latitude at 
the surface, and is rising still, so to speak. For as man 
has gone higher he has found his inferences rising too, 
and the limit would seem to be not yet. We see then 
that the air to which we thought ourselves so much in- 
debted, actually begins its kindly offices by shutting 
off two-thirds of what was coming to us. As it plays, 
however, something of the same trick to what tries to 
escape, we are really somewhat beholden to it after all. 
But not so much as has been thought. We used to be 
told that the Moon's temperature even at midday hardly 



192 THE EVOLUTION OF WORLDS 

rose above freezing, but Very has found it about 350° 
F., which even the most chilly of souls might find warm. 
By the late afternoon, however, he would need his over- 
coat, and no end of blankets subsequently, for during 
the long lunar night of fourteen days the temperature 
must fall appallingly low, to — 300° F, or less. 

As the determination of temperature is a vital one, 
not only to any organic existence, but even to inorganic 
conditions upon a planet, it behooves us to look care- 
fully into the question of the effective heat received from 
the Sun. Until recently the only criterion in the case 
was assumed to be distance from the illuminating source, 
about as efficient a mode of computation as estimating 
a Russian army by its official roll. For as we saw in 
our own case, not all that ought to ever gets to the front, 
to say nothing of what is lost there. Yet on this worse 
than guesswork astronomic text-books still assert as a 
fact that the temperature of other bodies — the Moon 
and Mars, for example — must be excessively low. 

Let us now examine into this most interesting prob- 
lem. It is intricate, of course, but I think you will find 
it more comprehensible than you imagine. Indeed, I 
shall be to blame if you do not. For if one knows his 
subject, he can always explain it, in untechnical lan- 
guage, technical terms being merely a sort of short- 
hand for the profession. The physical processes in- 
volved can be made clear without difficulty, although 



A PLANET'S HISTORY 



193 



their quantitative evaluation is less forthrightly demon- 
strable. Let me, then, give you an epitome of my in- 
vestigation of the subject. 

Consider a ray of light falling on a surface from the 



ATMOSPHERE 




Adventures of a heat ray. 



Sun. A part of it is reflected; that is, is instantly 
thrown off again. By this part the body shines and 
makes its show in the world, but gets no good itself. 
Another part is absorbed; this alone goes to heat 
the body. Now if the visible rays were all that ema- 
nated from the Sun, it would be strictly true, and a 



o 



194 THE EVOLUTION OF WORLDS 

pretty paradox for believers in the efficacy of distance, 
that what heated the planet was precisely what seemed 
not to do so. Unfortunately there are also invisible 
rays, and these, too, are in part reflected and in part 
absorbed, and their ratio is different from that of the 
visible ones. To appreciate them, Langley invented 
the bolometer, in which heat falling on a strip of metal 
produces a current of electricity registered by a gal- 
vanometer. By thus recording the heat received at 
different parts of the spectrum and at different heights 
in our atmosphere, he was able to find how much 
the air cut off. Very has since determined this still 
more accurately. By thus determining the depletion 
in the invisible part of the spectrum joined to what as- 
tronomy tells us of the loss in the visible part, we have 
a value for the whole amount. By knowing, then, the 
immediate brightness of a planet and approximately 
the amount of atmosphere it owns, we are enabled to 
judge how much heat it actually receives. This proves 
to be, in the case of Mars, more than twice as much 
as distance alone would lead us to infer. 

The second question is how much of this it retains. 
The temperature of a body at any moment is the balance 
struck between what it receives and what it radiates. 
If it gets rid of a great deal of its income, it will clearly 
be less hot than if it is miserly retentive. To find 
how much it radiates we may take the difference in 



A PLANET'S HISTORY 195 

temperature between sunset and sunrise, since during 
this interval the Earth receives no heat from the Sun. 
In the same way the efficacy of different atmospheric 
blankets may be judged. Thus the Earth parts with 
nine centigrade degrees' worth of its store on clear 
nights, and only four degrees' worth on cloudy ones, 
before morning. This is at sea-level. By going up a 
high mountain we get another set of depletions, and 
from this a relative scale for different atmospheric 
blankets. This is the principle, and we only have to 
fill out the skeleton of theory with appropriate num- 
bers to find how warm the body is. 

In doing so, we light on some interesting facts. Thus 
clouds reflect 72 per cent of the visible rays, letting 
through only 28 per cent of them. We feel chilly when 
a cloud passes over the Sun. On the other hand, slate 
reflects only 18 per cent of the visible rays, absorbing all 
the rest. This is why slate gets so much hotter in the 
Sun than chalk, and why men wear white in the tropics. 
White, indeed, is the best color to clothe one's self in the 
year around, except for the cold effect it has on the im- 
agination, for it keeps one's own heat in as well as keep- 
ing the Sun's out. The modest, self-obliterating, white 
winter habit of the polar hares not only enables them to 
keep still and escape notice, but keeps them warm while 
they wait. 

Astronomically, the effect is equally striking. Mars, 



196 THE EVOLUTION OF WORLDS 

for example, owing to being cloudless and of a duller 
hue, turns out to have a computed mean temperature 
nearly equal to the Earth's, — a theoretic deduction which 
the aspect of the planet most obligingly corroborates. 
It thus enjoys a comparatively genial old age. 

For what is specially instructive in planetary economy 
is that, on the whole, clear skies add more by what they 
let in than they subtract by what they let out. If the 
Earth had no clouds at all, its mean temperature would 
be higher than it is to-day. Thus as a planet ages a 
beneficent compensation is brought about, the Sun's 
heat increasing as its own gives out. Not that the 
foreign importation, however slight the duty levied on it 
by the air, ever fully makes up for the loss of the domes- 
tic article, but it tempers the refrigeration which inevi- 
tably occurs. 

The subject of refrigeration leads us to one of the most 
puzzling and vexed problems of geology: how to ac- 
count for the great Ice Age of which the manifest sign 
manuals both in Europe and in America have so in- 
trigued man since he began to read the riddle of the 
rocks. Upon this, also, planetology throws some light. 

If I needed an apology to the geologists for seeming 
again to trespass on their particular domain, I might 
refer to the astrocomico expositions put forward to 
account for the great Ice Age. 

We can all remember Croll's "Climate and Time," 



A PLANET'S HISTORY 197 

a book which can hardly be overpraised for its title and 
which had things worth reading inside, too. It had in 
consequence no inconsiderable vogue at one time. It 
undertook to account for glacial epochs on astronomic 
principles. It called in such grand cosmic conditions 
and dealt in such imposing periods of time that it fired 
fancy and almost compelled capitulation by the mere 
marshalling of its figurative array. Secular change in 
the eccentricity of the Earth's orbit, combined with 
progression in the orbital place of the winter's solstice, 
was supposed to have induced physical changes of cli- 
mate which accentuated the snowfall in the northern 
hemisphere and so caused extensive and permanent 
glaciation there. In other words, long, cold winters 
followed by short, hot summers in one hemisphere were 
credited with accumulating a perpetual snow sheet, 
such as short, warm winters and long, cold summers 
could not effect. 

Now it so happens that these astronomic conditions 
affecting the Earth several thousand years ago, are in 
process of action on one of our nearest planetary 
neighbors at the present time. The orbit of Mars 
is such that its present eccentricity is greater than 
what the Earth ever can have had, and the winter 
solstice of the planet's southern hemisphere falls 
within 23° of its aphelion point. We have then the 
conditions for glaciation if these are the astronomic 



198 THE EVOLUTION OF WORLDS 

ones supposed, and we should expect a southern polar 
cap, larger at its maximum and still more so, rela- 
tively, at its minimum, than in the opposite hemisphere. 
Let us now look at the facts, for we have now a knowl- 




Mars. 
north polar cap. 

At maximum full extent of white 
At minimum inner circle 



SOUTH POLAR CAP. 

At maximum white 
At minimum nothing 



edge of the Martian polar caps exceeding in some re- 
spects what we know of our own. The accompanying 
diagrams exhibit the state of things at a glance, the 
maximum and minimum of each cap being represented 
in a single picture and the two being placed side by side. 
It will be observed that the southern cap outdoes its 
antipodal counterpart at its maximum, showing that the 
longer, colder winter has its effect in snow or hoar-frost 
deposition. But, on the other hand, instead of excelling 



A PLANET'S HISTORY 199 

it at its minimum, which it should do to produce perma- 
nent glaciation, it so far falls short of its fellow that 
during the last opposition at which it could be well ob- 
served, it disappeared entirely. The short, hot summer, 
then, far exceeded in melting capacity that of the longer 
but colder one. 

Let us now suppose the precipitation to be increased, 
the winters and summers remaining both in length and 
temperature what they were before. The amount of 
snow which a summer of given length and warmth can 
dispose of is, roughly speaking, a definite quantity. For 
it depends to a great extent only on its amount of heat. 
The summer precipitation may be taken as offsetting 
itself in the two hemispheres alike. If, then, the snow- 
fall in the winter be for any reason increased daily in 
both, a time will come when the deposition due the longer 
winter of the one will exceed what its summer can melt 
relatively to the other, and a permanent glaciation re- 
sult in the hemisphere so circumstanced. Increased 
precipitation, then, not eccentricity of orbit, is the real 
cause of an Ice Age. And this astronomic deduction we 
owe not to theoretic conclusions, for which we lack the 
necessary quantitative data, but wholly to study of our 
neighbor in space. Had any one informed our geologic 
colleagues that they must look to the sky for definite 
information about the cause of an Ice Age, they would 
probably have been surprised. 



200 THE EVOLUTION OF WORLDS 

With this Martian information, received some years 
ago, it is pleasing now to see that Earthly knowledge is 
gradually catching up. For that increased precipitation 
could account for it, the evidence 6f pluvial eras in the 




Glacial map of Eurasia — after James Geikie. 

equatorial regions, contemporaneous with glacial periods, 
indicates. But another and probably the ^chief factor 
involved was not a generally increased precipitation, 
potent as that would be, but an increased snow deposit 
due to temporary elevation of the ground. 

For it now appears that there was no glacial epoch. 
Our early ideas inculcated by text-books at school re- 
ceived a rude shock when it appeared that the glacial 
epoch was not, as we had been led to believe, a polar 



A PLANET'S HISTORY 



201 



phenomenon at all, but a local affair which on the face 
of it had nothing to do with the pole. For investiga- 
tion has disclosed that instead of emanating from the 
pole southward, it 
proceeded from 
certain centres, de- 
scending thence in 
all directions, 
north as much as 
south. Thus 
there was a centre 
in Norway in 65° 
N. lat. and an- 
other in Scotland 
in 56° N. In 
North America 
there were three — 
the Labradorian 
in latitude 54° N., 

the Kerwatin to ^^^ showing the glaciated area of north 

America — the arrows indicating the di- 
the northwest of rection of ice movement — Chamberlin 

^ . and Salisbury. 

Hudson s Bay in 

latitude 62° N., and the Cordilleran along the Pacific 
coast in latitude 58° N. On the other hand, northern 
Siberia, the coldest region in the world, was not glaci- 
ated. That the ice flowed off these centres proves 
them to have been higher than the sides. But we have 




202 THE EVOLUTION OF WORLDS 

further evidence of their then great height from the 
fact that dead Httoral shells have been dredged from 
1333 fathoms in the North Atlantic, and the prolonga- 
tion under water of the fiords of Norv^ay and of land 
valleys in North America w^itness to the same sub- 
sidence since. 

But evidence refuses to stop here. The Alps were 
then more glaciated than they are now. So was Kili- 
manjaro and Ruwenzori on the equator; and finally at 
the same time more ice and snow existed round about 
the south pole than is the case to-day. Now this is 
really going too far even for the most ardent believers 
in the force of eccentricity. For if the astronomic 
causes postulated were true, they must have produced 
just the opposite action at the antipodes, to say nothing 
of the crux of being equally effective at the equator. 
The theory lies down like the ass between two burdens. 
Whichever load it chooses to saddle, it must perforce 
abandon the other. 

So it turns out that the Ice Age was not an Ice Age at 
all but an untoward elevation of certain spots, and is to 
be relegated to the same limbo of exaggeration of a 
local incident into a world-wide cataclysm as the deluge. 
That some geologists will still cling to their former belief 
I doubt not; for as the philosophic old lady remarked: 
"There always have been two factions on every sub- 
ject. Just as there are the suffragists and anti-suffra- 



A PLANET'S HISTORY 203 

gists now, so there were slaveholders and the anti- 
slavery people in my time; and even in the days of the 
deluge, there were the diluvians who were in favor of a 
flood and the antediluvians who were opposed to it." 
A tale which has a peculiarly scientific moral, as in 
science anti and ante seem often interchangeable terms. 

When I began the course of lectures that resulted in 
this volume, I labored under the apprehension that an 
account of cosmic physics might prove dull. It soon 
threatened to prove too startling. I therefore hasten to 
reassure the timid by saying that we are outgrowing ice 
ages and probably deluges. Elevations of the Earth's 
crust are likely to be less and less pronounced in the 
future, and meanwhile such as exist are being slowly 
worn down. Secondly, the Sun is sure to continue 
of much the same efficiency for many aeons to come. 
And lastly, the essential ingredient of both prodigies, 
water, is daily becoming more scarce. To this latter 
point we now turn, and perhaps when it is explained to 
him the reader may think that he has been rescued from 
one fate only to fall into the hands of another. 

Geology is necessarily limited in its scope to what 
has happened; planetology is not so circumscribed in 
its domain. It may indulge in prognostication of the 
future, and find countenance for its conclusions in the 
physiognomy of other worlds. Thus one of the things 
which it foresees is the relative drying up of our abode. 



204 THE EVOLUTION OF WORLDS 

To those whose studies have never led them off this 
earth, the fact that the oceans are slowly evaporating into 
space may seem as incredible as would, to one marooned 
on a desert island, the march of mankind in the mean- 
time. We live on an island in space, but can see some- 
thing of the islands about us, and our conception of what 
is coming to our limited habitat can be judged most 
surely by what we note has happened to others more 
advanced than ourselves. Just as we look at Jupiter to 
perceive some likeness of what we once were, the real 
image of which has travelled by this time far into the 
depths of space beyond possibility of recall, so must we 
look to the Moon or Mars if we desire to see some faint 
adumbration of the pass to which we are likely to come. 
For from their lack of size they should have preceded 
us on the road we are bound to travel. Now, both these 
worlds to-day are water-lacking, in whole or part; the 
Moon practically absolutely so. Mars so far as any 
oceans or seas are concerned. We should do wisely 
then to take note. But we have more definite informa- 
tion than simply their- present presentments. For both 
bear upon their faces marks of having held seas once 
upon a time. They were once, then, more as we are 
now. We cannot of course be sure, as we are unable to 
get near enough to scan their surfaces for signs of erosive 
action. But so far as we can make out, past seas best 
explain their appearance. 



A PLANET'S HISTORY 



205 



So sealike, indeed, was their look that the first astron- 
omers to note them took them unhesitatingly for water 
expanses. Thus the moment the telescope brought the 
Moon near enough for map making of it we find the 
dark patches at once 
designated as seas. 
The Sea of Serenity, 
the Sea of Showers, 
the Bay of Rainbows, 
speak still of what 
once was supposed to 
be the nature of the 
dark, smooth, lunar 
surfaces they name. 
Suggestively, indeed, 
in an opera glass do 
they seem to lap the land. The Lake of Dreams fore- 
shadowed what was eventually to be thought of them. 
With increasing optical approach the substance evapo- 
rated, but the form remained. It was speedily evident 
that there was no water there; yet the semblance of its 
repository still lurked in those shadows and suggests it- 
self to one scanning their surfaces to-day. If they be not 
old sea bottoms, they singularly mimic the reality in their 
smooth, sloping floors and their long, curving lines of 
beach. Their strange uniformity shows that something 
protected them from volcanic fury while the rest of the 




The Moon — Photographed at the 
Lowell Observatory. 



2o6 THE EVOLUTION OF WORLDS 

lunar face was being corrugated. This preservative 
points to some superincumbent pressure which can have 
been no other than water. Lava-flows on such a scale 
seem inadmissible. What these surfaces show and what 
they do not show alike hint them sea bottoms once 
upon a time. In the strange chalk-like hue of the lunar 
landscape they look like plaster of Paris death-masks of 
the former seas. 

A like history fell to the lot of the surface features of 
Mars. There too, as soon as the telescope revealed 
them and their permanency of place, the dark patches 
upon the planet's face were forthrightly taken for seas, 
and were so called : the Sea of the Sirens and the Great 
Red Sea. Such they long continued to be deemed. The 
seas of Mars held water in theory centuries after the 
idea of the lunar had vanished into air. At last, ruthless 
science pricked the pretty bubble analogy had pictured. 
Being so much farther off than the Moon, it was much 
later that their true character came out. Come out it 
has, though, within the last few years. Lines — some 
of the so-called canals — have been detected crossing 
the seas, lines persistent in place. This has effectually 
disposed of any water in them. But here again some- 
thing of semblance is left behind. They are still the 
darkest portions of the planet, and their tint changes 
in places with the progress of the planet's year. That 
their color is that of vegetation, and that its change obeys 



A PLANET'S HISTORY 207 

the seasons, stamp it for vegetation in fact. Thus these 
regions must be more humid than the rest of Mars. 
They must, therefore, be lower. That they are thus 
lower and possess a modicum of water to-day marks 
them out for the spots where seas would be, were there 
any seas to be. As we know of a vera causa which has 
for ages been tending to deplete them, extrapolation 
from what is now going on returns them the water they 
have lost and rehabilitates their ancient aquatic charac- 
ter. To the far-sight of inference, seas they again be- 
come in the morning of the ages long ago when Mars 
itself was young. 

Nor is this the end of the evidence. When we com- 
pare quantitatively the areas occupied by the quondam 
seas on Mars and on the Moon, we find reason to 
increase our confidence in our deduction. For the 
smaller body, the Moon, should have had less water 
relatively, at the time when the seas there were laid 
down, than the larger, Mars. Because from the mo- 
ment its mass began to collect, it was in process of part- 
ing with its gases, water-vapor among the rest, and, as 
we shall see more in detail in the next chapter, it had 
from the start less hold on them than Mars. Its oceans, 
therefore, should have been less extensive than the 
Martian ones. This is what the present lunar Mare 
seem to attest. They are less extended than the dark 
areas of Mars. A fact which becomes the more evident 



2o8 THE EVOLUTION OF WORLDS 

when we remember that the Moon has long turned 
the same face to the Earth. Her shape, therefore, has 
been that of an egg, with the apex pointing toward 
our world. Here the water would chiefly collect. The 
greater part of the seas she ever had should be on our 
side of her surface, the one she presents in perpetuity 
to our gaze. 

It is to the heavens that we must look for our surest 
information on such a cosmic point, because of the long 
perspective other bodies give us of our own career. 
Less conclusive, because dependent upon less time, is 
any evidence our globe can offer. Yet even from it we 
may learn something; if nothing else, that it does not 
contradict the story of the sky. To it, therefore, we 
return, quickened in apprehension by the sights we 
have elsewhere seen. 

The first thing our sharpened sense causes us to note 
is the spread of deserts even within historic times. Just 
as deserts show by their latitudinal girdling of the Earth 
their direct dependence upon the great system of plane- 
tary winds, as meteorologists recognizingly call them, 
so a study of the fringes of these belts discloses their en- 
croachment upon formerly less arid lands. The south- 
ern borders of the Mediterranean reveal this all the way 
from Carthage to Palestine. The disappearance of 
their former peoples, leaving these lands but scantily in- 
habited now, points to this; because other regions, as 



A PLANET'S HISTORY 209 

India, which still retain a waterful climate, are as popu- 
lous as ever. Much of this is doubtless due to the over- 
throw of dynasties and the ensuing lapse of irrigation, 
but query: Is it all ? For we have still more definite 
information in the drying up of the streams which have 
left the aqueducts of Carthage without continuation, 
as much to water on the one hand as to its drinkers on 
the other. Men may leave because of lack of water, 
but water does not leave because of dearth of men 
to drink. 

Recent search around the Caspian by Huntington 
has disclosed the like degeneration due to encroaching 
desertism there. Indeed, it is no chance coincidence 
that just where all the great nations thrived in the morn- 
ing of the historic times should be precisely where popu- 
lous peoples no longer exist. For neither increasing cold 
nor increasing heat is responsible for this, seeing that 
no general change has occurred in either. Nor were 
they particularly exposed to extermination by northern 
hordes of barbarians. Egypt as a world power died a 
natural death, and Babylonia too; but the common 
people died of thirst, indirect and unconscious and not 
wholly of their own choosing. Prehistoric records make 
this conclusion doubly sure, by lengthening the limit of 
our observation. Both extinct flora and extinct fauna 
tell the same tale. In the neighborhood of Cairo petrified 
forests attest that Egypt was not always a wiped slate, 



210 THE EVOLUTION OF WORLDS 

while the unearthed animals of the Fayum bear witness 
to water where no water is to-day. 

Anywhere we wander along these girdling belts we 
find the same story written for us to read. The great 




Petrified bridge, third petrified forest, near Adamana, Arizona — ■ 

Photograph by Harvey. 

deserts of New Mexico and Arizona show castellated 
structures far beyond the means of its present Indian 
population to inhabit. Yet this retrenchment occurred 
long before the white man came with his exterminating 
blight on everything he touched. Nor have we reason 
to suppose that it arose in consequence of invasion by 
other alien hordes. Individual communities may thus 
indeed have perished as the preservation of their domi- 
ciles intact leads us to infer, but all did not thus vanish 
from off the Earth. Here again humanity died or 
moved away because nature dried the sources of its 



A PLANET'S HISTORY 211 

supply. And here, as elsewhere, we find prehistoric rec- 
ord in the rocks of a once more smiling state of things, 
strengthening the testimony we deduce from man. 
The forests, crowning now only the greater heights, are 
but the shrinking residues of what once clothed the land. 
The well-named Arid Zone is becoming more so every 
day. 

If from the land evidence of drying up we turn to 
the marine, we see the same shrinkage at work. It has 
even been discovered in a lowering of the ocean bed, but 
as this may so easily be disputed, we turn to one aspect of 
the situation which cannot so easily be gainsaid, — the 
bodies of water that have been cut off. That the Dead 
Sea, the Caspian, the Great Salt Lake, are slowly but 
surely giving way to land, is patent. If the climate at 
least were not more arid than before this could not 
occur; but more than this, if the ocean were not on the 
whole shrinking, there would be no tendency to leave 
such arms of itself behind to shrivel up. That the 
ocean basins are deepening is possible, but we know 
of one depletion which is not replaced — evaporation 
into space; and of another bound to come — withdrawal 
into fissures when the earth shall cease to be too hot. 

This gradual withdrawal of the water may seem an 
unpleasant one to contemplate, but like most things it 
has its silver lining in the hope it holds out that some- 
time there shall be no more sea. Those of us who 



212 THE EVOLUTION OF WORLDS 

detest the constant going down to the sea in ships hardly 
more than the occasional going down with them, can 
take a crumb of comfort in the thought. Unfortunately 
it partakes of a somewhat far-off realization in our dis- 
tant descendants, coming a little too late to be of ma- 
terial advantage to ourselves. 

But let me not leave the reader wholly disconsolate. 
For another thought we can take with us in closing our 
sketch of so much of the Earth's life as brings it well 
down to to-day, — the thought that it has grown for us 
a steadily better place to contemplate from the earliest 
eras to the present time. Indeed, with innate prescience 
we forbore to appear till the prospect did prove pleas- 
ing. Finally, we may palliate prognostication by con- 
sidering that if its future seem a thought less attractive, 
we, at least, shall not be there to see. 



CHAPTER VIII 



DEATH OF A WORLD 



EVERYTHING around us on this Earth we see is 
subject to one inevitable cycle of birth, growth, 
decay. Nothing that begins but comes at last to end. 
Not less is this true of the Earth as a whole and of each 
of its sister planets. Though our own lives are too brief 
even to mark the slow nearing to that eventual goal, the 
past history of the Earth written in its rocks and the 
present aspects of the several planets that circle similarly 
round the Sun alike assure us of the course of aging as 
certainly as if time, with all it brings about, passed in 
one long procession before our very eyes. 

Death is a distressing thing to contemplate under any 
circumstances, and not less so to a philosopher when 
that of a whole world is concerned. To think that this 
fair globe with all it has brought forth must lapse in 
time to nothingness; that the generations of men shall 
cease to be, their very records obliterated, is something 
to strike a chill into the heart of the most callous and 
numb endeavor at its core. That aeons must roll away 
before that final day is to the mind of the far-seeing 
no consolation for the end. Not only that we shall pass, 

213 



214 THE EVOLUTION OF WORLDS 

but that everything to show we ever were shall perish 
too, seems an extinction too overpowering for words. 

But vain regret avails not to change the universe's 
course. What is concerns us and what will be too. 
From facing it we cannot turn away. We may alleviate 
its poignancy by the thought that our interest is after 
all remote, affecting chiefly descendants we shall never 
know, and commend to ourselves the altruistic exam- 
ple so nobly set us by doctors of medicine who, on the 
demise of others at which — and possibly to which — 
they have themselves assisted, show a fortitude not 
easily surpassed, a fortitude extending even to their 
bills. If they can act thus unshaken at sight of their 
contemporaries, we should not fall behind them in 
heroism toward posterity. 

Having in our last chapter run the gantlet of the ge- 
ologists, we are in some sort fortified to face death — 
in a world — in this. The more so that we have some 
millenniums of respite before the execution of the de- 
cree. By the death of a planet we may designate that 
stage when all change on its surface, save disintegration, 
ceases. For then all we know as life in its manifold 
manifestations is at an end. To this it may come by 
many paths. For a planet, like a man, is exposed to 
death from a variety of untoward events. 

Of these the one least likely to occur is death by ac- 
cident. This, celestially speaking, is anything which 



DEATH OF A WORLD 215 

may happen to the solar system from without, and is 
of the nature of an unforeseen catastrophe. Our Sun 
might, as we remarked, be run into. For so far as we 
know at present the stars are moving among themselves 
without any too careful regard for one another. The 
swarm may be circling a central Sun as Andre states, 
but the individual stars behave more like the random 
particles of a gas with licensed freedom to collide; 
whereas we may liken the members of the solar system 
to molecules in the solid state held to a centre from 
which they can never greatly depart. Their motions 
thus afford a sense of security lacking in the universe 
at large. 

Such an accident, a collision actual or virtual with 
another sun, would probably occur with some dark star; 
of which we sketched the ultimate results in our first 
chapter. The immediate ones would be of a most dis- 
astrous kind. For prefatory to the new birth would 
be the dissolution to make such resurrection possible. 
Destruction might come direct, or indirectly through 
the Sun. For though the Sun would be the tramp's 
objective point, we might inadvertently find ourselves 
in the way. The choice would be purely academic ; be- 
tween being powdered, or deorbited and burnt up. 

So remote is this contingency that it need cause us 
no immediate alarm, as I carefully pointed out. But so 
strong is the instinct of self-preservation and so pleasur- 



2i6 THE EVOLUTION OF WORLDS 

able the sensation of spreading appalling news, that the 
pressof America, and incidentally Europe, took fire, with 
the result, so I have been written, that by the time the 
pictured catastrophe reached the Pacific " it had as- 
sumed the dimensions of a first magnitude fact." 

This is the first way in which our world may come by 
its death. It is possible, but unlikely. For our Earth, 
long before that, is morally certain to perish otherwise. 

The second mode is one, incident to the very consti- 
tution of our solar system. It follows as a direct out- 
come of that system's mechanical evolution, and may 
be properly designated, therefore, as due to natural 
causes. It might be diagnosed as death by paralysis. 
For such it resembles in human beings, palsy of indi- 
vidual movement afflicting a planet instead of a man. . 

Tidal friction is the slow undermining cause ; a force 
which is constantly at work in the action of every body 
in the universe upon every other. As we previously 
explained, the pull of one mass upon another is inevi- 
tably differential. Not only is the second drawn in its 
entirety toward the first, falling literally as it circles 
round, but the nearer parts are drawn more than the 
centre and the centre more than those farthest away. 
We may liken the result to a stretched rotating rubber 
ball, with, however, one important difference, — that each 
layer is more or less free to shear over the others. The 
bulge, solicited by the rotation to keep up, by the dis- 



DEATH OF A WORLD 217 

turber to lag behind, is torn two ways, and the friction 
acts as a break upon the body's rotation, tending first to 
turn it over if it be rotating backward and then to slow 
it down till the body presents the same face in perpetuity 
to its primary. The tides are the bulge, not simply 
those superficial ones which we observe in our oceans, 
and know to be so strong, but substantial ones of the 
whole body which we must conceive thus as egg-shaped 
through the action that goes on — the long diameter 
of the egg pointing somewhat ahead of the line joining 
its centre to the distorting mass. All the bodies in the 
solar system are thus really egg-shaped, though the de- 
formation is so slight as to escape detection observa- 
tionally. The knowledge is an instance of how much 
more perceptive the brain is than the eye. For we are 
certain of the fact, and yet to see it with our present 
means is impossible, and may long remain so. 

Two concomitant symptoms follow the friction of the 
tidal ansae : a shift of the plane in which the rotation 
takes place, and a loss of speed in the spin itself. The 
first tends to bring the plane of rotation down to the 
orbital plane, with rotation and revolution in the same 
sense. This effect takes place quicker than the other, 
and in consequence different stages may be noted in the 
creeping paralysis by which the body is finally overcome. 
Loss of seasons characterizes the first. For the coinci- 
dence of the two planes means invariability in the Sun's 



21 8 THE EVOLUTION OF WORLDS 

declination throughout the year for a given latitude. 
This reduces all its days to one dead level in which 
summer and winter, spring and autumn, are always and 
everywhere the same. There is thus a return at the end 
of the planet's career to an uneventful condition rem- 
iniscent of its start; a senility in planets comparable 
to second childhood in man. 

In large planets this outgrowing of seasons occurs 
before they have any, while the planet is yet cloud- 
wrapped. Such planets know nothing of some attri- 
butes of youth, like those unfortunate men who never 
were boys; just as reversely the meteorites are boys 
that never grew up. For if the planet be large, the ac- 
tion of the tidal forces is proportionately more power- 
ful; while on the other hand the self-aging of the 
planet is greatly prolonged, and thus it may come about 
that the former process outstrips the latter to the missing 
of seasons entirely. This is sure to be the case with 
Jupiter, as the equator has already got down to within 
3° of the orbit, and threatens to be the case with Saturn. 
These bodies, then, when they shall have put off their 
swaddling clothes of cloud, will wake to climates without 
seasons; globes where conditions are always the same 
on the same belts of latitude, and on which these alter 
progressively from equator to pole. Variety other than 
diurnal is thus excluded from their surfaces and from 
their skies. For the Sun and stars will rise always the 



DEATH OF A WORLD 219 

same, in punctual obedience only to the slowly shifting 
year. 

The next stage of deprivation is the parting with the 
day. Although the day disappears, the result is too 
much day or too little, depending on where you choose 
to consider yourself upon the afflicted orb. For 
tidal friction proceeds to lengthen the twenty-four or 
other hours first to weeks, then months, then years, and 
at last to infinity; thus bringing the sun to a stock-still 
on the meridian, to flood one side of the world with 
perpetual day and plunge the other in eternal night. 

Which of these two hemispheres would be the worse 
abode, is matter of personal predilection; dust or 
glacier, deserts both. Everlasting unshielded noon 
would cause a wind circulation from all points of the 
enlightened periphery to the centre, whence a funnel- 
shaped current would rise to overflow back into the 
antipodes, thence to return by the horizon again. As 
the night side would be several hundred degrees at least 
colder than the noon one, all the moisture would be 
evaporated on the sunlit hemisphere, to be carried round 
and deposited as ice on the other, there to stay. Life 
would be either toasted or frappe. A Sahara backed 
by polar regions would be the obverse and the reverse 
of the shield. 

The reader may deem the picture a fancy sketch 
which possibly may not appeal to him. Nevertheless, 



220 THE EVOLUTION OF WORLDS 

it not only is possible, but one which has overtaken our 
nearest of neighbors. To this pass the Mater Amorum, 
Venus herself, has already been brought. She betrays 
it by the wrinkles which modern observation has re- 




October 15, 1896. February 12, 1897. March 26, 1897. 

Venus — Drawings by Dr. Lowell showing agreement at different 

distances. 

vealed upon her face. Innocent critics, with a gallantry 
one would hardly have credited them, — which shows 
how one may wrong even the humblest of creatures, — 
have denied the existence of these marks of age, on the 
chivalrous a priori assumption that it could not possi- 
bly be true because never seen before. Their negation, 
in naive ignorance of the facts, partakes the logic 
of the gallant captain, who, when asked by a lady 
to guess her age, replied: "'Pon my word, I haven't the 



DEATH OF A WORLD 221 

slightest idea," hastily adding, " But you don't look it ! " 
Less commendable than this conventional nescience, 
but unfortunately more to the point, is the evidence of 
prying scientific curiosity. Shrewdly divined as much 
as detected by Schiaparelli, made more certain by the 
crow's-feet disclosed at Flagstaff, and corroborated by 
the testimony of the spectroscope there, her isochronism 
of rotation and revolution lies beyond a doubt. At- 
traction to her lord has conquered at last her who was 
the cynosure of all. Venus, in her old age, stares for- 
ever at the Sun, and we all know how ill an aging 
beauty can support a garish light. 

Mercury has been brought to a like pass. This was 
evident even before the facts came out about Venus, for 
Venus, true to her instincts, shields herself with a veil 
of air which largely baffles man's too curious gaze. 
Mercury, on the other hand, offers no objection to ob- 
servation. When looked for at the proper time, his 
markings are quite distinct, dark, broken lines suggesting 
cracks. Schiaparelli, again, was the first to perceive 
the true state of the case, and his observations were in- 
dependently confirmed and extended at Flagstaff in 
1896. In so doing the latter disclosed a very interesting 
fact. It was evident that the markings held in general 
a definite fixed position upon the illuminated part of 
the disk, showing that the planet kept the same face 
always to the Sun. But systematic observation, con- 



222 THE EVOLUTION OF WORLDS 



tinued day after day for weeks, disclosed a curious shift, 
which, though sHght, was unmistakable. Upon thought 
the cause suggested itself, and on being subjected to 
calculation proved equal to such accounting. In this 




A.B, 




A.B. 




Diagram of libration in longitude due to rotation. 

singular systematic sway stood revealed the libration 
in longitude caused by the eccentricity of the planet's 
orbit. 

Mercury revolves about the Sun in an ellipse more 
eccentric than that of any other principal planet. At 
times he is half as far off again from him as he is at 
others. When near, he travels faster than when far. 



1 








1 


HI ^' 7 .H 






1 


1 


^^LV_^j_^JH 






1 








1 


H^^3B 


^^^^^^^ **;:;^^^2r^fl ^ 


^^^^^^^^^H 


1 









DEATH OF A WORLD 223 

For both reasons, nearness and speed, his angular 
revolution about the Sun varies greatly from point to 
point according to v^here he finds himself in his orbit. 
His rotation, however, is necessarily uniform. For even 
the Sun has no power at once to change the enormous 
moment of momentum of his axial spin. In conse- 
quence, at times his angular velocity of revolution 
gains on his rotation, at other times loses, both coming 
out together at the end of a complete Mercurial year. 
The result is a superb rhythmic oscillation, a true 
mejcurial pendulum compensated by celestial laws to 
perfect isochronism of swing. 

The outward sign of this shows in the movement of 
the markings. To observers in space like ourselves, 
the planet seems to sway his head as he travels along 
his orbit. For weeks he turns his face, as shown by the 
markings on it, more and more over to the left; then 
turns it back again as far over to the right. It is as if 
he were looking furtively around as he hastens over his 
planetary path. 

Venus, of course, is equally subject to this law of 
distraction, but owing to the almost perfect circularity 
of her orbit she is less visibly affected. In fact, it is not 
possible to detect her lapse from a fixed regard to the 
Sun. At most it is no more than a glance out of the 
corner of her eyes — her slight deviation from perfect 
rectitude of demeanor. Knowledge of the laws gov- 



224 THE EVOLUTION OF WORLDS 

erning such action alone permits us to recognize its 
occurrence. 

Mercury and Venus are the only planets as yet that 
turn a constant face to their overruling lord. The 
reason for this appears when one goes into the matter 
analytically. The tidal force is not the direct pull of 
the Sun on a particle of the body, but the difference in 
the pulls upon a particle at the centre and one at the 
circumference. Being differential, it depends directly 
upon the radius of the distorted body and inversely upon 
the third power of its distance away. As the space 
through which the force acts is proportional to the force 
itself, the effect is as the squares of the quantities 
mentioned, or, inversely, as the sixth power of the 
distance and as the square of the body's radius. The 
result thus proves greatest on the planets nearest to the 
Sun, and diminishes rapidly as we pass outward from 
him. If, then, the solar force had had time enough to 
produce its effects, it would be first in Mercury and then 
in Venus that it should be seen. And this is precisely 
where we observe it. 

The Moon presents us a well-known case of such filial 
regard, resulting in permanent incompetency of action 
on its own account. It turns always the same face to 
us, following us about with the mute attention of a dog 
to its master. Here again the libration may be detected, 
for no dog but makes excursions on the road. This case 



DEATH OF A WORLD 225 

differs from those of Mercury and Venus in that the body 
to which the regard is paid is not also the dispenser of 
hght and warmth. In consequence, though the side of 
the Moon with which we are presented remains always 




Moon — full and half, photographed at the Lowell Observatory. 

the same, we do not always see it; the light creeping 
over it with the progress of the lunation, from new to 
full. On this account the worst that happens to our 
Moon in its old age is that its day becomes its month. 
Our Moon is not peculiar in having its day and its 
month the same. On the contrary, it is now the rule 
with satellites thus to protract their days. So far as we 
can observe, all the large satellites of Jupiter turn the 
same face to him; those of Saturn pay him a like re- 
gard; while about those of Uranus and Neptune we 
are too far off to tell. Their direct respect for their 
primary, with only secondary recognition of the Sun, 
keeps them from the full consequences of their fatal 

Q 



226 THE EVOLUTION OF WORLDS 

yielding to attraction. It is bad enough to have the 
day half a month long, but worse to have one that never 
ends, or, still worse, perpetual night. 

In our diagnosis of the cause of death in planets, we 
now pass from paralysis to heart failure. For so we 
may speak of the next affection which ends in their 
taking off, since it is due to want of circulation and lack 
of breath. It comes of a planet's losing first its oceans 
and then its air. 

To understand how this distressing condition comes 
about, we must consider one of the interesting scientific 
legacies of the nineteenth century to the twentieth : the 
kinetic theory of gases. 

The kinetic theory of gases supposes them to be made 
up of minute particles all alike, which are perfectly 
elastic and are travelling hither and thither at great 
speeds in practically straight lines. In consequence, 
these are forever colliding among themselves, giving 
and taking velocities with bewildering rapidity, resulting 
in a state of confusion calculated to drive a computer 
mad. Somebody has likened a quiet bit of air to a 
boiler full of furious bees madly bent on getting out. 
The simile flatters the bees. To follow the vicissi- 
tudes of any one molecule in this hurly-burly would 
be out of the question ; still more, it would seem, that of 
all of them at once. Yet no less Herculean a task con- 
fronts us. To find out about their motions, we are 



DEATH OF A WORLD 



227 



therefore driven to what is called the statistical method 
of inquiry, — which is simply a branch of the doctrine 
of probabilities. It is the method by which we learn 
how many people are going to catch cold in Boston next 




Illustrating molecular motion in a gas (black molecules here con- 
sidered AT rest). 

week when we know nothing about the people, or about 
colds, or about catching them. At first sight it might 
seem as if we could never discover anything in this 
hopelessly ignorant way, and as if we had almost better 
call in a doctor. But in the multitude of colds — not 
of counsellors — lies wisdom. So in other things not 
hygienic. As you cannot possibly divine, for instance, 
what each boy in town is going to do during the year. 



228 THE EVOLUTION OF WORLDS 

nor what is his make of mind, how can you say whether 
he will accidentally discharge a firearm and shoot his 
playmate or not ! And yet if you take all the boys of 
Boston, you can predict to a nicety how many will thus 
let off a gun and ** not know that it was loaded." 

In this only genuine method of prophecy, complete 
ignorance of all the actual facts, we are able without 
knowing anything whatever about each of the mole- 
cules to predicate a good deal about them all. To begin 
with, the pressure a gas exerts upon the sides of a vessel 
containing it must be the bombardment the sides re- 
ceive from the little molecules; and the heating due 
this rain of blows, or the temperature to which the ves- 
sel is raised, must measure their energy of translation. 
On this supposition it is found that the laws of Avoga- 
dro and of Boyle are perfectly accounted for, besides 
many more properties of gases which the theory ex- 
plains, and as nothing yet has been encountered seri- 
ously contradicting it, we may consider it as almost as 
surely correct as the theory of gravitation. To three 
great geniuses of the last century we owe this remarkable 
discovery — Clausius, Clerk Maxwell, and Boltzmann. 

By determining the density of a gas at a given tem- 
perature and under a given pressure, we can find by the 
statistical method the average speed of its molecules. 
It depends on the most probable distribution of their 
energy. For hydrogen at the temperature of melting 



DEATH OF A WORLD 



229 



ice, and under atmospheric pressure, this speed proves 
to be a Httle over a mile a second — - a speed, curiously 
enough, which is to that of light almost exactly as centi- 
metres to miles. But some of the molecules are going 




Distribution of molecular velocities in a gas. 

at speeds much above the mean; fewer and fewer as 
the speed gets higher. Just how many there are for 
any assigned speed, we can calculate by the same in- 
genious application of unknown quantities. 

These speeds have been found for a temperature of 
freezing, and as the speed varies as the square root of 
the absolute temperature, we might suppose that when 
an adventurous or lucky molecule arrived at practically 
the limit of the atmosphere, where the cold is intense, it 
would become numbly sluggish. But let us consider 



230 THE EVOLUTION OF WORLDS 

this. When we enclose a gas in a cooler vessel, the 
molecules bombard the sides more than they are bom- 
barded back. In consequence, they lose energy; as we 
say, are cooled. But in free air if a molecule be fortunate 
enough to elude its neighbors, there is nothing to take 
away its motion but the ether through radiation, and 
this is a very slow process. Thus the escaping fugitive 
must arrive at the confines of the air with the speed it 
had at its last encounter. We reach, then, this result: 
In space there is no such thing as temperature; tem- 
perature being simply the aggregate effect of molecular 
temperament. The reason we should consider it un- 
commonly cold up there is that fewer molecules would 
strike us. Quantity, therefore, in our estimation re- 
places quality, — a possible substitution which also ac- 
counts for some reputations, literary or otherwise. The 
only forces which could affect this lonely molecule would 
be the heating by the Sun, the repellent force of light, 
and gravity. 

Now the speed which gravity on the Earth can con- 
trol is 6.9 miles a second. It can impart this to a body 
falling freely to it from infinite space, and can therefore 
annul it on the way up, and no more. If, then, any of 
the molecules reach the outer boundary of the air going 
at more than this speed, they will pass beyond the 
Earth's power to restrain. They will become little 
rovers in space on their own account, and dart off on 



DEATH OF A WORLD 231 

interstellar travels of their own. This extension of the 
kinetic theory and of the consequent voyages of the 
molecules is due to Dr. Johnstone Stoney, who has 
since, humorously enough, tried to stop the very balls 
he set rolling. First thoughts are usually the best, after 
all. 

As among the molecules some are already travelling 
at speeds in excess of this critical velocity, molecules 
must constantly be attaining to this emancipation, and 
thus be leaving the Earth for good. In consequence 
there is a steady drain upon its gaseous covering. 
Furthermore, as we know from comets' tails, the re- 
pellent power of the light-waves, what we may call the 
levity of light, much exceeds upon such volatile va- 
grants the heat excitement or even the gravity of the 
Sun, so that we arrive at this interesting conclusion — 
their escape is best effected under cover of the night. 

Again, the heavier the gas, the less its molecular speed 
at a given temperature, because its kinetic energy which 
measures that temperature is one-half the molecule's 
mass into the square of its speed. Thus their ponder- 
osity prevents as many of them from following their 
more agile cousins of a different constitution. So that 
the lighter gases are sooner gone. Water-vapor leaves 
before oxygen. Nor is there any escape from this es- 
cape of the gases. It may take excessively long, but 
go they must until a solitary individual who happens 



232 THE EVOLUTION OF WORLDS 

to have had the wrong end of the last colHsion is alone 
left hopelessly behind. 

Another factor also is concerned. The smaller the 
planet, the lower the utmost velocity it can control, and 
the quicker, therefore, it must lose its atmosphere. For 
a greater number of molecules must at every instant 
reach the releasing speed. Thus those bodies that 
are little shall, perforce, have less to cover themselves 
withal. 

Now this inevitable depletion of their atmospheric 
envelopes, the aspects of the various planets strikingly 
attest. They do so in most exemplary fashion, accord- 
ing to law. The larger, the major planets, as we have al- 
ready remarked, have a perfect plethora of atmosphere, 
more than we at least know what to do with in the way 
of cataloguing yet. The medium-sized, like our own 
Earth, have a very comfortable amount; Mars, an un- 
comfortable one, as we consider, and the smallest none 
at all. All the smaller bodies of our system are thus 
painfully deprived so far as we can discover. We are 
certain of it in the case. of our Moon and Mercury, 
the only ones we can see well enough to be sure. In 
further evidence it has been shown at the Yerkes and at 
FlagstaflF that no perceptible effect of air betrays itself 
in the spectroscopic imprint of the rings of Saturn, those 
tiny satellites of his, and very recently a spectrogram of 
Ganymede, Jupiter's third moon, made at Flagstaff for 



DEATH OF A WORLD 233 

the purpose by Mr. E. C. Slipher has proved equally 
void of atmospheric hint. 

With the loss of water and of air, all possibility of 
development departs. Not only must every organism 
die, but even the inorganic can no longer change its 
state. In the extinction thus not only of inhabitants 
but of the habitat that made them possible, occurs a 
curious inversion of the order we are familiar with in 
the life-history of organisms. In planets it is the 
grandchildren that die first, then the children, and 
lastly their surviving parent. And this is not acci- 
dental, but inevitably consequent upon their respective 
origins. For the ofF-spring, as we may spell it with a 
hyphen, of any cosmic mass is of necessity smaller than 
that from which it issued. Being smaller, it must age 
quicker. In the natural order of events, then, its end 
must be reached first. 

Such has been the course taken, or still taking, by the 
bodies of our solar family. The latest generation has 
already succumbed to this ebbing of vitality with time. 
Every one of the satellites of the planets — those of 
Neptune, Uranus, Saturn, Jupiter, and our own Moon — 
is practically dead ; born so the smaller which never 
were alive. Our own Moon carries its decrepitude on 
its face. To all intents and purposes its life is past; 
and that it had at one time a very fiery existence, the 
great lunar craters amply testify. It is now, for all its 



234 THE EVOLUTION OF WORLDS 

flooding with radiance our winter nights, the Hfeless 
statue of its former self. 

The same inevitable end, in default of others, is 
now overtaking the planetary group. Its approach is 
stamped on the face of Mars. There we see a world 
dying of exhaustion. The signs of it are legible in the 
markings we descry. How long before its work is done, 
we ignore. But that it is a matter of time only, our 
study of the laws of the inexorable lead us to conclude. 
Mars has been spared the fate of Mercury and Venus 
to perish by this other form of planetary death. 

Last in our enumeration of the causes by which the 
end of a world may be brought about, because the last 
to occur in order of time, is the extinction of the Sun 
itself. Certain to come and conclude the solar system's 
history as the abode of life, if all the others should by 
any chance fail to precede it, it fittingly forms the climax, 
grand in its very quietude, of all that went before. 

By the same physical laws that caused our Earth 
once to be hot, the Sun shines to-day. Only its greater 
size has given it a life and a brilliancy denied to smaller 
orbs. The falling together of the scattered particles of 
which it is composed, caused, and still is causing, the 
dazzling splendor it emits. And so long as it remains 
gaseous, its temperature must increase, in spite of its 
lavish expenditure of heat, as Homer Lane discovered 
forty years ago. 



DEATH OF A WORLD 235 

But the Sun's store of heat, immense as it is to-day, 
and continued as it is bound to be for untold aeons by 
means of contraction of its globe upon itself, and pos- 
sibly by other causes, must some day give out. From its 
present gaseous condition it must gradually but event- 
ually contract to a solid one, and this in turn radiate 
all its heat into space. Slowly its lustre must dim as 
it becomes incapable of replenishing its supply of 
motive power by further shrinkage in size. Fitfully, 
probably, like Mira Ceti to-day, it will show tem- 
porary bursts of splendor as if striving to regain the 
brightness it had lost, only to sink after each effort 
into more and more impotent senility. At last some 
day must come, if we may talk of days at all when 
the great event occurs when all days shall be blotted 
out, that the last flicker shall grow extinct in the orb 
that for so long has made the hearth of the whole 
system. For, presciently enough, the Latin word focus 
means hearth, and the body which includes within it the 
focus about which all the planets revolve also con- 
stitutes the hearth from which they all are lighted and 
warmed. 

When this ultimate moment arrives and the last spark 
of solar energy goes out, the Sun will have reverted once 
more to what it was when the cataclysm of the foretime 
stranger awoke it into activity. It will again be the 
dark body it was when our peering into the past first 



236 THE EVOLUTION OF WORLDS 

descries it down the far vista of unrecorded time. 
Ghostlike it will travel through space, unknown, un- 
heralded, till another collision shall cause it to take a 
place again among the bright company of heaven. 
Thus, in our account of the career of a solar system, we 
began by seeing with the mind's eye a dark body travel- 
ling incognito in space, and a dark body we find our- 
selves again contemplating at the end. 

In this kaleidoscopic biograph of the solar system's 
life, each picture dissolves into its successor by the falling 
together of its parts to fresh adjustments of stability, as 
in that instrument of pleasure which so witched our 
childish wonder in early youth. Just as when a com- 
bination had proved so pretty, once gone, to our sor- 
row no turning of the handle could ever bring it back, 
so in the march of worlds no retrace is possible of steps 
that once are past. Inexorable permutations lead from 
one state to the next, till the last of all be reached. 

Yet, unlike our childhood's toy, reasoning can conjure 
up beside the present picture far vistas of what pre- 
ceded it and of what is yet to come. Hidden from 
thought only by the distraction of the day, as the uni- 
verse to sight lies hid by the day's overpowering glare, 
both come out on its withdrawal till we wonder we never 
gazed before. Our own surroundings shut out the 
glories that lie beyond. Our veil of atmosphere cloaks 
them from our view. But wait, as an astronomer, till 



DEATH OF A WORLD 237 

the Sun sinks behind the hills and his gorgeous gold of 
parting fades to amber amid the tender tapestry of trees. 
The very air takes on a meaning which the flood of day 
had swamped. Seen itself, no longer imperfectly seen 
through, it wakes to semi-sentient existence, a spirit 
come to life aloft to shield us from the too immediate 
vacancy of space. The perfumes of the soil, the trees, 
the flowers, steal out to it, as the twilight glow itself ex- 
hales to heaven. In the hushed quiet of the gloaming 
Earth holds her breath, prescient of a revelation 
to come. 

Then as the half-light deepens, the universe appears. 
One by one the company of heaven stand forth to 
human sight. Venus first in all her glory brightens 
amid the dying splendor of the west, growing in lustre 
as her setting fades. From mid-heaven the Moon lets 
fall a sheen of silvery light, the ghostly mantle of her 
ghostlike self, over the silent Earth. Eastward Jupiter, 
like some great lantern of the system's central sweep, 
swings upward from the twilight bow to take possession 
of the night. Beyond lies Saturn, or Uranus perchance 
dim with distance, measuring still greater span. All in 
order in their several place the noble cortege of the Sun 
is exposed to view, seen now by the courtesy of his with- 
drawal, backgrounded against the immensity of space. 
Great worlds, these separate attendants, and yet as noth- 
ings in the void where stare the silent stars, huge suns 



238 THE EVOLUTION OF WORLDS 

themselves with retinues unseen, so vast the distances 
'twixt us and them. 

No less a revelation awaits the opening of the shut- 
ters of the mind. If night discloses glimpses of the 
great beyond, knowledge invests it with a meaning un- 
folding and extending as acquaintance grows. Sight 
is human; insight seems divine. To know those points 
of light for other worlds themselves, worlds the tele- 
scope approaches as the years advance, while study 
reconstructs their past and visions forth their future, 
is to be made free of the heritage of heaven. Time 
opens t;o us as space expands. We stand upon the 
Earth, but in the sky, a vital portion not only of our 
globe, but of all of which it, too, forms part. To feel it 
is to enter upon another life ; and if to realization of its 
beauty, its grandeur, and its sublimity of thought these 
chapters of its history have proved in any wise the 
portal, they have not been penned in vain. 



NOTES 



NOTES 



Meteor Orbits 

If the space of the solar system be equally filled with 
meteors throughout, or if they diminish as one goes out 
from the Sun according to any rational law, their average 
speed of encounter with the Earth would be nearly para- 
bolic. 

If they were travelling in orbits like those of the short- 
period comets, that is with their aphelia at Jupiter's orbit 
and their perihelia at or within the Earth's, their major 
axes would lie between 6.2 and 5.2. If we suppose their 
perihelion distances to be equally distributed according to 
distance, we have for the mean a major axis of 5.7. Their 
velocity, then, at the point where they cross the Earth's 
track would be given by , 

Vi 2.85/ 

in which />t = 18.5^ in miles per second 

= 342.25, 
whence v = 23.76 in miles per second. 

Suppose them to be approaching the Earth indifferently 
from all directions. 

At sunset the zenith faces the Earth's quit ; at sunrise 
the Earth's goal. Let be the real angle of the meteor's 
approach reckoned from the Earth's quit ; 6^ the apparent 
angle due to compounding the meteor's velocity-direction 

R 241 



242 THE EVOLUTION OF WORLDS 

with that of the Earth. Then those approaching it at any 
angle less than that which makes 0^ = 90° will be visible 
at sunset ; those at a greater angle, at sunrise. The angle 
6^ is given by the relation, 

/» ^ 

cos c/i = +-> 

in which a is the Earth's velocity, x the meteor's, and 0^ 
is reckoned from the Earth's quit. 

The portion of the celestial dome covered at sunset is, 
therefore, 

J^ sin e-dO' d(t), 

where (/> is the azimuth, 

I sin ' dd - d(j). 

If the meteors have direct motion only, can never 
exceed 90°, and the limits become, 

I sin 6 ' dO' dcf), 

and for sunrise, 1 I sin - d9 - defy. 

The mean inclination at sunset is 

Jf*9^ /»3C0= 
I I 6^'sme-dd' d(t> 
n •A 



J^0 /'360° » 

I sin e'de-d(l> 
•^o 



in which 6^ must be expressed in terms of 6, etc. 

From this it appears that the relative number of bodies, 
travelling in all directions and at parabolic speed, which 
the Earth would encounter at sunrise and sunset respect- 
ively would be : — 

sunrise . . . . . . . . 5-^ 

sunset i.o 

and with the speed of the short-period comets, 



NOTES 



243 



sunrise 8.0 

sunset 1.0 

If, however, the bodies were all moving in the same sense 
as the Earth, i.e. direct, the ratios would be : — 







Parabolic 
Speed 


Speed of Short 
Period Comets 


Speed of Actual Short- 
Period Comets about 
Jupiter 


Sunrise 

Sunset 


2.4 
1.0 


3-5 
1.0 


3-3 

1.0 



As the actual number encountered is between 2 and 3 to i, 
we see that the greater part must be travelling in the same 
sense as the Earth, since they come indifferently at all 
altitudes from the plane of her orbit. 



Densities of the Planets 

The densities of the principal planets, so far as we can 
determine them at present, the density of water being 
unity, are: — 

Mercury . . . 3.65 Jupiter . . . . 1.33 

Venus .... 5.36 Saturn .... o 72 

Earth .... 5.53 Uranus .... 1.22 

Moon .... 3.32 Neptune . . . i.ii 

Mars .... 3.93 mean 1.09 

mean 4.36 Sun 1.38 

The second decimal place is not to be considered as 
anything but an indication. 



Variation in Spectroscopic Shift 

In the case of a body reflecting light, the shift differs 
from that for a body emitting it. If the planet be on the 



244 THE EVOLUTION OF WORLDS 

further side of the Sun, the approaching rim advances both 
toward the Sun and toward the Earth, thus doubUng the 
shift. The receding rim recedes in Hke manner. At 
elongation the rims approach or recede with regard to the 
Earth, but not the Sun, and the shift is single as for 
emission. At inferior conjunction rotational approach to 
the Earth implies rotational recession from the Sun, and the 
two effects cancel. 

4 

On the Planets' Orbital Tilts 

The tilts of the plane of rotation of the Sun and of the 
orbits of the several planets to the dynamical plane of the 
system tabulated are : — 

Sun 7° Asteroids various 

Mercury 6° 14' Jupiter 20' 

Venus 2° 4' Saturn .56' 

Earth 1° 41' Uranus . 1° 2' 

Mars 1° 38' Neptune 43' 

where, in the determination of that plane, the latest values 
of the masses of the planets and the rotations of the Sun, 
Jupiter, and Saturn have been taken into account. 

These tilts suggest something, doubtless, but it is by no 
means clear what it is they suggest. They are just as com- 
patible with a giving off from a slowly condensing nebula 
as with an origin by shock. The greater inclinations of 
Mercury and Venus may be due to their late birth from 
the central mass without the necessity of a cataclysm, the 
rotation of that central mass out of the general plane being 
caused by the consensus of the motions of the particles from 
which it was formed. The accordance of the larger planet- 
ary masses with the dynamical plane of the system would 
necessarily result from their great aggregations. So that 
this, too, is quite possible without shock. 



NOTES 



245 



Planets and their Satellite Systems 

If we compute the speeds of satellites about their prima- 
ries in the solar system and compare them with the veloci- 
ties in their orbits of the planets themselves, a striking 
parallelism stands displayed between the several systems. 
This is shown in the following table of them : 







Mean Speed, Miles 
A Second 


Parabolic Speed 
AT Orbit 


Ratio Speed 

Sat. about 


• 


of Primary in 
Orbit 


of Satellite 
about Primary 


Miles a second 


Primary to 

Planet's Speed 

in Orbit 






V 


Z' 






Jupiter 


. 


8.1 


- 


II.5 




Sat. 


I 




10.7 




1.32 




2 




8.5 




1.05 




3 




6.7 




0.83 




4 




51 




0.63 


Saturn 


. 


6.0 




8.5 






I 




9.0 




1.50 




2 




7-9 




I-3I 




3 




8.2 




1.36 




4 




6.3 




1.05 


4 


5 




5-3 




0.89 




6 




3-5 




0.59 




8 




2.0 




0-34 


Uranus 


. 


4.2 




5.9 






I 




3-5 




0.82 




2 




2.9 




0.70 




3 




2-3 




0.54 




4 




2.0 




0.47 


Neptune . 


• 


3-4 




4.8 






I 




2.7 




0.81 



The relations here disclosed are too systematic to be the 
result of chance. 



246 THE EVOLUTION OF WORLDS 

The orbits of all these satellites have no perceptible 
eccentricity independent of perturbation except lapetus, of 
which the eccentricity is about .03. 

In view of the various cosmogonies which have been 
advanced for the genesis of the solar system it is interest- 
ing to note what these speeds imply as to the effect upon 
the satellites of the impact of particles circulating in the 
interplanetary spaces at the time the system evolved. To 
simplify the question we shall suppose — which is suffi- 
ciently near the truth — that the planets move in circles, 
the interplanetary particles in orbits of any eccentricity. 

Taking the Sun's mass as unity, the distance R of any 
given planet from the Sun also as unity, let the planet's 
mass be represented by M and the radius of its satellite's 
orbit, supposed circular, as r. We have for the space 
velocity of the satellite on the sunward side of the planet, 
calling that of the planet in its orbit V and that of the 
satellite in its orbit round the planet v, 



For a particle, the semi-major axis of whose orbit is a^ 
and which shall encounter the satellite, 

the velocity is v^^i— ]• 

That no effect shall be produced by the impact of these 
two bodies, their velocities must be equal, or 



a/I 



l-v?-v 



R ^ r ^ R — r a-^ 

As R — r = a-^{i -\- e) for the point of impact if the 
particle be wholly within the orbit of the planet and e the 

eccentricity of its orbit, we find e = 2\ approx. 



NOTES 



247 



for the case of no action, the other terms being insensible 

R 

for the satelhtes in the table, since in all r < 

400 

Supposing, now, the particles within the orbit of the 

planet to be equally distributed according to their major 

axes, then as the velocity of any one of them, taking 

R ~ r = R approx. as unity, is 






the mean velocity of all of those which may encounter the 
satellite is, at the point of collision. 



X 



1(2 <2j— I) 



da^ 



a. 



£da^ 



— 2 



(2 a^ — a-^y = log 1(2 a^ — i)' + V2 ^^^ 

V2 



= 0.754; 

that is, just over three-quarters of the planet's speed in its 
orbit. 

If we suppose the particles to be equally distributed in 
space, we shall have more with a given major axis in pro- 
portion to that axis, and our integral will become 

I {2a^— ifa-^da-^ 



j a^da^ 



^ 1 {2a^-a^y- 



8 



6V; 



log 



{2a^-a^^ 



+ V2 • a^ — 



2V2 



0.792 of the planet's orbital speed. 



248 THE EVOLUTION OF WORLDS 

The speed v, then, at which a satellite must be moving 
round the planet to have the same velocity as the average 
particle within the planet's orbit, is 

V — "(J^^ V. 

This velocity is, for the several planets : — 





Distribution of 

Particles as their 

Major Axes 


Distribution of 

Particles Equal 

IN Space 




Miles a second 


Miles a second 


Jupiter 

Saturn 

Uranus 

Neptune . 


2.0 

1-5 

I.O 

0.8 


1.6 

1.2 
0.9 
0.7 



If the satellite be moving in its orbit less fast than this, 
its space-speed will exceed that of the average particle; it 
will strike the particle at its own rear and be accelerated 
by the collision. If faster, the particle will strike it in 
front and retard it in its motion round its primary. 

From the table it appears that all the large satellites of 
all the planets have an orbital speed round their primaries 
exceeding those in either column. In consequence, all of 
them must have been retarded during their formation by 
the impact of interplanetary particles and forced nearer 
their primaries than would otherwise have been the case ; 
and this whether the particles were distributed more densely 

toward the Sun, as — , or were equally strewn throughout. 

Cl-\ 

For interplanetary particles whose orbits lie without the 
particular planet's path the mean speed is the parabolic at 
the planet's distance, given in the third column of the table. 
This is the case on either supposition of distribution. The 



NOTES 



249 



orbital speed of the satellite which shall not be affected by 
collisions with them is, for the several planets : — 



Jupiter 
Saturn 
Uranus 
Neptune 



Miles a Second 



3-4 
2.5 

1.7 
1.4 



All the satellites but lapetus have orbital speeds exceed- 
ing this, and consequently are retarded also by these 
particles. 

For particles crossing the orbit (2) the mean velocity would 
be practically parabolic, 1.4, even if the distribution were as 

— , / being the distance from the Sun. The effect would de- 

r 

pend upon the angle of approach and in the mean give a 
greater velocity for the particle than for the satellite within 
the orbit, a less one without ; retarding the satellite in 
both cases. Thus the total effect of all the particles 
encountering the large satellites is to retard them and to 
tend to make them hug their primary. 

For retrograde satellites the velocities of impact with 
inside and outside particles moving direct are respectively: 





Inside 


Outside 


Jupiter 


2.0 + Z> 

1.5 + -^ 
1.0+2^ 

0.8 + V 


V + 3.4 

V + 2.5 

V + 1.7 

V + 1.4 


Saturn 

Uranus . 

Neptune 



In both cases the impact tends to check the satellite. 
Comparing with these the velocities of impact for direct 
satellites in a direct plenum : — 



250 THE EVOLUTION OF WORLDS 



Jupiter . 
Saturn . 
Uranus • 
Neptune 



Inside 



2.0 — V 
1.5 — V 
\.0 — V 
0.8 — V 



Outside 



34- "v 

2.5 — V 

l.J — V 

1.4 — V 



the signs being taken positive when the motion is direct, 
we see that retrograde satellites would be more arrested 
than direct ones with the same orbital speed round the 
primary. 

In a plenum of direct moving particles, then, the force 
tending to stop the satellite and bring it down upon the 
planet is greater for retrograde satellites than for direct 
ones. 

If, therefore, the positions of the satellites have been 
controlled by the impact of interplanetary particles, the 
retrograde satellites should be found nearer their planets 
than the direct ones. 



On the Induced Circularity of Orbits through 

Collision 

Since the moment of momentum is the velocity into the 
perpendicular upon its direction, in the time dt it is : — 

vj^dt = /idt = r^dO. 

The whole moment of momentum from perihelion to 
perihelion is therefore : — 



X 



r'^de = "^ y' ^ ) 

360° 

— e sin 



I -\- e cos 



+ 



(I -.2) 



^tan-N- 
2\i V ^ I 



-\-e 



e ^ e 

tan 



NOTES 251 

= 2 ircP' • ( I — e^Y-> 

which is twice the area of the elHpse. 

The energy in the elUpse during an interval dt is 



- mv^dt = - m/jL { ]dt 

2 2 \r a) 



from the well-known equation for the velocity in a focal 
conic. The integral of this for the whole ellipse is 

02 ^^ 2 h \ a) 



1 1 



Since 



\ rd6= I ^^^= V- tan M \ tan - 

J rau Ji^eco^e {i - e^f \^i + e 2) 

and I r^dO is given above. 

By collision a part of this energy is lost, being converted 
into heat. The major axis, a, is, therefore, shortened. 
But from the expression 2 ira/'' • (i — e^y for the moment of 
momentum we see that this is greatest when e is least. 
If, therefore, a is diminished, e must also be diminished, or 
the moment of momentum would be lessened, which is 
impossible. 

7 

Capture of Satellites 

See has recently shown {Astr. Nach. No. 4341-42) that 
a particle moving through a resisting medium under the 
attraction of two bodies revolving round one another in 
circles may eventually be captured by one of them though 
originally under the domination of both. The argument 
consists in introducing the effect of a resisting medium 



252 THE EVOLUTION OF WORLDS 

upon the motion in the space permitted by Jacobi's inte- 
gral, following Darwin's examination of this space. In 
the actual case of nature the effect is much more compli- 
cated, and at present is not capable of exact solution for 
masses other than indefinitely small, even supposing 
circular orbits for the chief bodies. It may, however, 
explain the curious relation shown in the arrangement of 
the direct and retrograde movement of satellites. 



INDEX 



INDEX 



Abnormality, the survival of original 

state, 144, 146. 
Absorption in spectrum, 

planetary, 52, 161. 

of Uranus, 118. 

of Jupiter, 152. 

of Saturn, 152. 
Achilles, 94. 
Adams, 119, 121. 
Adams, Mr. J. C, 123-126. 
Agassiz, 41. 
Airy, 121, 123. 
Albedo, 

of dark star, 27. 

of Mercury, 62, 73-75. 

of Venus, 73-75. 

of Moon, 75. 

of Jupiter, 104, 105. 

of Saturn, 109. 

of Uranus, 116. 

of Neptune, 168. 

of clouds, 195. 
Algol, 3. 

American Academy, 125. 
Amphibians, first record of, 188, 
Anderson, Dr. Thomas D., 8, 12. 
Andre, 265. 
Andromeda, great nebula in, 10, 20, 21. 

constitution disclosed by spectro- 
scope, 45, 48. 
Apex of Sun's way, 26. 
Arago, 121. 
Asteroids, 39, 60, 61, 94-102. 

domain of, 94. 

diminutive size, 94, loi. 

number, 94, 10 1. 

peculiar discovery of, 95-98. 

never formed part of a pristine whole, 
98. 

where thickest, 98. 

formation of large planet from, pre- 
vented, 98, 99. 

mid-course between planets and 
comets, 100. 



Asteroids — cont. 

shape of, loi, 102. 

mammoth meteorites, 102. 

mark transition between inner and 
outer planets, 102. 
Atmosphere, 

spectrographic study of, 53, 54, 161. 

Mercury deprived of, 71, 75, 232. 

reflecting power, 75. 

of Venus, 75. 

Moon deprived of, 75, 232. 

thin on Mars, 75, 91, 232. 

of Uranus, enormous, 117, 118, 232. 

of Neptune, vast, 118, 232. 

of Jupiter, 166, 232. 

depletion of, 231-233. 

none on Ganymede, 232, 233. 

of Saturn, 232. 

lacking in Saturn's rings, 232. 
Avogadro, 228. 
Axes of planets, 

systematic righting of, 132. 

tilts accounted for, 146. 

B 

Babinet, 147. 

Backland, 68. 

Ball, Sir Robert, 145. 

Barrande, M., 178. 

Belopolski, 87. 

Bessel, 120, 121. 

Blandet, M., 175, 176. 

Bode, 95, 119. 

Bode's law, 96, 100, 119, 122, 126. 

Bolometer, 194. 

Bolton, Mr. Scriven, 103, 105, 106. 

Boltzmann, 228. 

Bose, 157. 

Bouvard, Alexis, 120, 121. 

Boyle, 228. 

Bradley, 68. 

c 

Cambrian era, 178. 
Cambridge Observatory, 123. 
Campbell, 9. 



255 



256 



INDEX 



Carboniferous period, 179. 

Cassini, 76, 162. 

Celestial mechanics, 28, 94, 155. 

Ceres, 10 1, 

Challis, 123. 

Chemistry, indebted to the stars, 160. 

Clausius, 228. 

Clerke, Miss, 9, 164. 

Climate, advent of, 185. 

Clouds, 

none on Venus, 75. 

of Jupiter not ordered as ours, 107, 
163, 167. 

Uranus wrapped in, 168. 

Neptune wrapped in, 168. 

Earth once wrapped in, 170, 171, 178. 
Collision of dark star with Sun, 25, 215. 

warning of, 26-29. 

disturbances previous to, 29, 30. 

rarity of event, 30. 
Collisions between meteorites of a 
flock, II, 49. 

causing light, 49, 50. 
Columbus, 188. 
Comets, ;^^, 61. 

members of solar system, 34, 35. 

orbits of, 61, 100. 
Commensurability of orbital period, 99, 

III. 
Congruities of solar system, 128-137. 

deviations from, 62, 100, loi, 130, 

i3i» 141. 

specify mode of evolution, 137. 
Convection currents, 219. 

in atmosphere of Venus, 80. 
Copeland, Dr. 7. 
Copernican system, 58. 
Copernicus, 62. 
Cosmic action, i, 22, 184. 
CroU, 196. 
Cuticle of star, effect of impact on, 1 1 . 



D 



Dana, 177, 186, 189. 
Dark stars, 

origin, 2. 

number, 2, 25. 

evidence of, 3-5. 

collision of, 10, 11. 

rendered visible, 26. 
Darwin, 62, 138, Notes 252. 
Day, 

lengthened to infinity, 70, 219. 



Day — cont. 

none on Venus, 8^. 

Jovian, 163. 

first appreciation of, 186. 

coincides with month, on satellites, 225. 
Death of a planet, 

defined, 214. 

catastrophic cause, 215, 216. 

due to tidal retardation of rotation, 
216-219. 

outcome of loss of oceans and air, 
226, 2^^. 

caused by extinction of Sun itself, 234. 
Density, 

of dark star, 27. 

of planets, 51, Notes 243. 

of Mercury, 63, 64. 

of Venus, 90. 

of Jupiter, 103, 117. 

of Uranus, 115. 
Deserts, increase of, on Earth, 208-211. 
Devonian era, 187. 
Dhurmsala meteorite, 41. 
Diameter, 

of Mercury, 63, 64, 66, 67. 

of Venus, 90. 

of Earth, 90. 

of Mars, 91. 

of satellites of Mars, 92. 

of Jupiter, 103. 

of Uranus, 11 5-1 17. 
Dust, in atmosphere of Venus, 75. 



Earth, 

characteristics, not universal, 90, 91, 

155- 
evolved from a nebula, 149. 
internal heat, 150. 
early surface temperature, 160, 169, 

170. 
once cloud-wrapped, 170, 171, 178. 
solid surface formed, 171. 
hot seas of, 171, 172. 
self-sustained, 182. 
study of, within province of astron- 
omy, 184. 
ceased to be self-centred, 187. 
Sun becomes dominant factor in 
organic life of, 190. 
Earth shine, 82. 
Eccentricity, orbital, 

of Mercury, 63, 65, 69, 222. 
of asteroids, erratic, 100, 10 1. 



INDEX 



257 



Eccentricity, orbital — cont. 

of satellites, increases with distance 
from primary, 134. 
Eclipsing binaries, 3, 4. 
Ejectum from nova, 5, 16. 

rate of regression, 16. 
Elemental substances, 159. 

in Sun, 159. 

once in Earth, 160. 

discovery of, in stars, 161, 162. 
Ellipticity, 

of Jupiter, 103. 

of Saturn, 109. 

of Uranus, 115. 
Encke, 68. 
Energy, 

conservation of, 140, 150, 151. 

dissipation, 140-142. 

conditions for a minimum, 142. 
Eros, fluctuation of light of, gives evi- 
dence of form, loi, 102. 
Evolution, 153. 

white nebulae in process of, 49. 

rounded out, 56. 

of solar family, 100. 

evidence of, in solar system, 117. 

manner of, lessens energy, 141. 
Evolution, chemical, 155, 173. 

universal, 156. 

temperature conducive to, 157, 158. 

attendant upon cooling, 158, 162. 

steps in, shown by spectroscope, 161. 
Evolution, physical, 155, 162. 

induced by cooling, 162. 



Fabry, 34. 

Fauna, 178, 179, 187, 
Faye, 175, 176. 

Flagstaff, Arizona, 52, 66, 68, 79, ^^^ 
89, 92, 106, no, 221, 232. 
clear and steady air of, 66, 86. 
Flamstead, 119. 
Fleming, Mrs., 7. 
Flemming, 120, 121. 
Flora, of paleologic times, 177. 
French Academy, 122. 

G 

Galle, Dr., 122, 123, 125. 

Gases, 

peculiar to nebulae, 11, 16. 
occluded in meteorites, 42, 43. 
in atmospheres of planets, 53-55. 



Gauss, 34, 96, 97. 
Geikie, 160, 177, 189. 
Geology, 

relation to astronomy, 173, 174, 183, 
184. 

scope of, 174, 203. 
Geysers, avenues to earlier state, 160. 
Goodricke, 3. 

H 

Hakluyt, 188. 

Harvard College Observatory, 8, 12. 

Heat, 

molecular motion, 150, 157, 230. 

the result of evolving, 153. 

the preface to higher evolution, 153, 
156. 

laws governing amount of, 190. 

atmosphere keeps out, as well as 
stores, 191. 

effective, received from Sun, 192-194. 

invisible rays, 194. 

retained, 194-196. 

radiated, 194-196. 
Heat of condensation of Earth, 

accuses concourse of particles, 151. 

evaluated, 151, 152. 

sufficient for geologic phenomena, 152, 
Hector, 94. 
Helmholtz, 151. 
Hencke, 98. 

Herschel, Sir John, 122. 
Herschel, Sir William, 96, 114, 162. 
Hertha, periodic variability, 102. 
Hipparchus, 5. 
Holden, 9. 

Hubbard, Professor, 124. 
Huggins, 52. 
Humphreys, 10. 
Huntington, 209. 



Ice Age, 196. 

not of orbital occasioning, 197-199. 
increased precipitation, the cause, 

199, 200. 
a local affair, 200-202. 
Irradiation, affecting diameter of Mer- 
cury, 66, 68. 



Jacobi, Notes 252. 
Julius, Professor, 10. 
Juno, loi. 



258 



INDEX 



Jupiter, 103-108. 

not solid, 104, 107. 

a semi-sun, 105, 108, 152, 166, 
167. 

white spots of, 106. 
Jupiter, "great red spot" of, 164. 

time of rotation, 164. 

a vast uprush of heated vapor, 165, 
166. 
Jupiter's belts, 

secular progression, 104. 

rotate at different speeds, 104, 162, 
163. 

color, 104. 

wisps across, 105, 106. 

bright ones, cloud, 163, 167. 

spectrographic study of, 166. 



Kapteyn, 14. 

Keeler, 19, 52, no. 

Kepler, 6. 

Kinetic theory of gases, 226, 228. 

corollary of, 54. 

extension of, 230, 231. 
Kirkwood, Professor, 35, 



Lagrange, 94, 97. 

Lalande, 123, 124. 

Lane, Homer, 234. 

Langley, 191, 194. 

Laplace, 34, no, 127, 129, 131, 132, 

138. 139. 147, 152, 175- 
Laplacian cosmos, 129, 130. 
false congruities of, 131-133. 
annular genesis, disproved, 138, 

139- 

original "fire mist" of, impossible, 
138. 
Lapparent, de, 173-176, 183, 189. 
Lemonnier, 115, 119. 
Leonard, Miss, 79. 
Leverrier, 119, 121-126. 
Lexell, 115. 
Libration in longitude, 

of Mercury, 65, 69, 70, 222, 223. 

causes true day, 70, 71. 

of Venus, inappreciable, 83, 223. 

of Moon, 224. 
Lick Observatory, 13, 14, 
Lockyer, 48. 
Lowell Observatory, 65, 74. 



M 

Major planets, 

gaseous, 117. 

constitution of, differs from Sun or 
Earth, r6i. 

types of early planetary stages, 162. 

self-centred and self-sustained, 168. 
Man, immanent, 159. 
Mars, 

polar caps, 198. 

canals in dark regions, 206, 207. 

dying of exhaustion, 234. 
Mass, 

of Mercury, 63, 64, 68. 

of Mars, 91. 

of Jupiter, 103. 

arrangement of, in solar system, 135- 
137, 148. 
Massachusetts Institute of Technology, 

134, 184. 
Mauvais, 125. 

Maxwell, Clerk, no, 113, 228. 
Mayer, 119, 151. 
Mendeleeff, 161. 
Mercury, 62-73. 

time of rotation and revolution the 
same, 65, 69. 

axis stands plumb to orbit, 70. 

turns same face to the Sun, 70, 72, 
134, 221. 

surface markings, 72, 221. 

color, 72. 
Meteorites, 31, 35, 36. 

cosmic bodies, 32, ^;^. 

relation to shooting stars, 36. 

members of solar system, 36. 

composition, 40-44, 55. 

fused by friction with atmosphere, 
40. 

temperature, 41, 55. 

fragments of a dark body, 44. 

link past to present, 44, 56, 57, 130. 
Meteors, 

orbits of, 36, 39, Notes 241-243. 

visibility of, 38. 
Meteor-streams, 33, 61. 

first recognition of, 34. 

disintegrated comets, 34. 
Michelson, 10. 
Milham, Professor, 99. 
Mira Ceti, 235. 
Mohler, 10. 
Molecular speeds, gaseous, 228-231. 

critical velocity, 230, 231. 



INDEX 



259 



Molecule, organic, power in its insta- 
bility, 160. 
Moment of momentum, 140, Notes 250. 

cause of original, 130. 
Moment of momentum, conservation 
of, 140. 

applied to solar system, 141-143. 
Momentum, 140. 
Monch, Mr., 10. 
Moon, 

turns same face to Earth, 134, 208, 
224, 225. 

once fiery, now dead, 233, 234. 
Mountains, none on Mars, 91. 
Muller, 73, 74, 104, 105, 116. 

N 

Naval Observatory at Washington, 122. 
Nebulae, 

origin of, 10, 11. 

amorphous, 18, 44. 

planetary, 18. 

spectrum of amorphous, 45. 
Nebulae, spiral, 17-25, 44. 

evolved from disrupted stars, 10-15. 

relation to novae, 14-16. 

corpuscular character of, 15, 16. 

knots and patches of, 15. 

most common, 19, 20. 

two- armed, 20, 25. 

central neucleus, globular, 21. 

not due to explosive action, 22, 23, 25. 

not caused by disintegration, 24, 25. 

cause of development, 24, 25. 

spectrum of, 45-48. 

composed of flocks of meteorites, 48, 
49. 

constitution established by spectro- 
scope, 49, 50. 
Nebular hypotheses, 173. 
Neologic times, clearing of sky in, 185. 
Neptune, 118. 

rotates backward, 118. 

owes discovery to- mathematical tri- 
umph, I 19-126. 

faint belts on, 168. 

further advanced than giant planets, 
168. 
Newcomb, 67. 
Newton, Professor, 36, 42. 
Newton, Sir Isaac, 34. 
Nova Aurigae, 7, 8, 12. 

history chronicled by its spectrum, 
8, 9. 



Nova Cygni, 7. 
Novae, 6, 7. 

origin 5, 10. 

first chronicled, 5. 

spectroscopic study of, 7. 
Nova Persei, 7. 

history of, 12-15. 

O 

Oceans, 

none on Mars, 91. 

evaporation of, 204. 

basins of, on Moon, 204-208. 

basins of, on Mars, 206, 207. 
Olbers, 97. 

Olmstead, Professor, ^^. 
Orbital distance, 

of Mercury, 62. 

of Venus, 73. 

of Mars, 91. 

of Eros, 94. 

of Saturn, 108. 
Orbital tilts, 

of asteroids, erratic, 100, loi. 

of satellites of Uranus, 116. 

of planets, substantially the same, 
129-131, Notes 244. 

deviation from rule, by Mercury, 131. 

of satellites, increase with distance 
from primary, 133, 134. 
Orbits, 

determining factors, 35. 

rendered more circular by collisions, 
141-143. Notes 250, 251. 

made more conformant to general 
plane by collisions, 1 41-143. 
Orion, great nebula in, 18. 



Paleologic times, 

much warmth and little light in, 172. 
fallacies in geologists' expositions of, 

174-176. 
climate continuous, 177, 186. 
seas warm, 177, 178. 
explained by cloud envelope, 178. 
corroboration of explanation, 187, 

179. 
excessive rain in, 185, 186. 
passage into Neologic, essentially 
astronomic, 185. 
Pallas, loi. 
Parabolic speed at orbit. Notes 245. 



26o 



INDEX 



Patroclus, 94. 
Peirce, no, 125, 126. 
Perrine, 15. 
Perrotin, 116. 
Perturbations, 

in motion of planets, heralding a 
catastrophe, 28, 30. 

reflected, 63. 

mass of planet determined by, 68. 

of asteroids by Jupiter, 98, 99. 

restrictive action of, 99. 

the fashioning force of planetary or- 
bits, 99, 100. 

of rings of Saturn by satellites, iii, 
112. 

of Uranus lead to discovery of Nep- 
tune, 121-126. 
Petersen, Dr., 123. 
Photometric determinations, 92, 93. 

background, the fundamental factor 
in, 92, 93. 
Piazzi, 96. 
Pilgrim Star, 5, 6. 
Planetary astronomy, advance in, 59, 

60. 
Planetology, 203. 

defined, 173, 174, 
Planets, 61. 

knots in spiral nebulae, 25, 139. 

developed by agglomeration, 143, 

149, 151, 152. 
Pliny, 5. 

Plutonic rocks, 160. 
Pluvial eras, contemporaneous with 

glacial, 200. 
Polyp corals, in paleologic times, 

186. 
Pristine motion of planetary particles, 
retrograde, 144. 
superfluous energy in, 145. 
unstable, 145. 
Ptolemaic system, 58. 



R 



Refrigeration, tempered by loss of 

cloud, 196. 
Revolutions, 

of shooting stars, 39. 
of asteroids, direct like planets. 100. 
planetary, in same sense, 129, 130. 
outermost satellites, retrograde, 132. 
of sateUites explained. 146, 147, 
Notes 252. 



Ritchey, 14. 

Roberts, Dr., 20. 

Roche, Edouard, no, 

Rosse, Lord, 17. 

Rotation of planets, 131, 132. 

systematic righting of axes, 132. 

initially, retrograde, 146. 
Rotation period, 

of Venus, spectrographically deter- 
mined, 83, 85-90. 

of Mars, spectrographically deter- 
mined, 88, 89. 

of Jupiter, spectrographically de- 
termined, 89. 

of Uranus, 116. 
Royal Observatory, Edinburgh, 7. 



Satellites, 61. 

of Mars, 92. 

of Saturn, 108, 112. 

of Uranus, 116. 

solid, 117. 

of Neptune, 118 

turn same face to primaries, 134, 147, 
148, 225. 

latest discoveries in regard to mo- 
tions of, 146. 

origin of, 147. 

death of, before planet, 233. 

impact of interplanetary particles on. 
Notes 246-250. 

capture of. Notes 251, 252. 
Saturn, 108-114. 

belts of, 109, 168. 

inherent light, 109, 152. 
Saturn's rings, 109-114. 

mechanical marvel of, not early ap- 
preciated, no. 

discrete particles, no, 135. 

knots upon, no-113. 

not flat, but tores, 111-114. 

show devolution — not pristine state 
of solar system, 138, 139. 

once a congeries, 139. 
Schaeberle, 9. 
Schiaparelli, 34, 36, 64-66, 69, 76, 77, 

221. 
Schroeter, 65, 77. 
Seasons, 

loss of, 71, 83, 217, 218. 

begin with clearing of sky, 185. 

fully developed, 189. 
See, Notes 251. 



INDEX 



261 



Seeliger, 10. 
Shooting-stars, 33, 35. 

radiant of, 33, 36. 

members of solar system, 36-40. 

tiny planets, 39. 
Siderite, 36. 
Silurian era, 178. 

Sirona, periodic variability of, 102. 
Sky, cause of clearing, 187. 
SUpher, Dr. V. M., 52, 79, 83, 86, 88, 

89, 117, 161, 166. 
Slipher, Mr. E. C, 79, 2;^;^. 
Solar constant, 191. 
Solar system, 

evolved from a dark star, 44. 

evidence of origin, 51, 130. 

characteristics of, 60-62. 

evolutionarily one, 62. 

gap in progression of orbital dis- 
tances, 95-100. 

bodies of, egg-shaped, 217. 
Specific gravity, of stone and iron, 44. 
Spectroscope, 7, 84. 
Spectroscopic shift, 84. 

determining velocity, 3. 

in Nova Aurigae, 9. 

produced by great pressure, 10, 13. 

produced by anomalous refraction, 
10. 

produced by change of density, 10, 

explained, 85. 

variation in. Notes 243, 244. 
Spectrum, 

of Nova Persei, 12, 13. 

nebular, 13, 16, 45-48. 

peculiarities of nebular, explained, 

50- 
photographic extension of, 52, 117, 

161. 
of major planets, 52, 53, 161. 
of belts of Jupiter, 166. 
Spiral structure, implies rotation com- 
bined vi^ith motion out or in, 22. 
Stability of a system, condition for, 140, 

141. 
Stoney, Dr. Johnstone, 231. 
Struve, 109. 
Suess, 179. 
Sun, 

original slow rotation of the, 130. 
heat of, 234, 235. 
reversion to a dark star, 235, 236. 
Sun spots, 104, 166. 



Temperature, 

of Moon, 191, 192. 
of Mars, 192, 194, 196. 
defined, 230. 

no such thing as, in space, 230. 
Tercidina, periodic variability of, 

102. 
Tertiary times, entrance of color with, 

189, 190. 
Tidal action, 143-147, 216-218. 
causes loss of energy, 144. 
inoperative, 144, 145, 147. 
changes retrograde rotation of planet 

to direct, 145-147, 217. 
on satellites, 147. 
slows down spin, 148, 217. 
brings plane of rotation down to 

orbital plane, 217. 
lengthens day to infinity, 219. 
analytically expressed, 224. 
greatest on planets near Sun, 135, 
224. 
Tidal action, disruptive, 130. 

exemplified by spiral nebulae, 24, 

25- 
hinted at, by meteorites, 55. 
theory corroborated by densities of 

planets, 51. 
theory corroborated by atmospheres 

of planets, 52-55. 
on comets, 139. 
cause of Saturn's rings, 139. 
Tisserand, 68. 
Titius, 95. 
Todd, 68. 
Trees, deciduous, first appearance of, 

189. 
Trilobites, blindness of, 178, 179. 
Twining, S3- 
Tycho Brahe, 5. 



U 



Uranus, 114-118. 

history of discovery, 114, 115, 119. 

a ball of vapor, 115, 117. 

belts of, 115, 116, 168. 

tilt of axis to ecliptic, great, 115. 

spectroscopic revelations of, 117, 

118. 
in an early amorphous state, 118. 
further advanced than the giant 

planets, 168. 



262 



INDEX 



Velocity, 

of Mercury in orbit, 63. 

of satellites about primary, Notes 

245- 
of major planets, in orbit, Notes 245. 

Venus, 73-90. 

surface markings, 74, 77, 79, 80, 

83, 220, 221. 
brilliancy due to cloudless atmos- 
phere, 75. 
importance of rotation period, 75, 

76. 
turns same face to the Sun, 77-80, 

134, 220, 221. 
ice on the night side, causes ashen 
light, 82. 
Very, Professor, 16, 191, 192, 194. 
Vesta, loi. 
Vogel, 52. 

Volcanoes, avenues to earlier state, 160. 
Von Zach, 96. 



W 



Walker, Mr., 123, 124. 
Water, 

becoming more scarce, 203, 204, 211. 

lacking on Moon, 204. 
Water-vapor, 

in atmosphere of Jupiter, 53. 

in atmosphere of Mars, 91, 161. 

smaller planet has less hold on, 207. 
Williams, Mr. Stanley, 103. 
Witt, de, 94. 
Wolf, Dr., 13. 
Wolf, Max, 94. 
Wolf-Rayet stars, 13, 48. 
Wright, 13, 43. 



Year, of Uranus, 116. 
Yerkes Observatory, 232. 
Young, 46. 



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